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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 Notes on the Generic Block Layer Rewrite in Linux 2.5
2 =====================================================
3
4Notes Written on Jan 15, 2002:
5 Jens Axboe <axboe@suse.de>
6 Suparna Bhattacharya <suparna@in.ibm.com>
7
8Last Updated May 2, 2002
9September 2003: Updated I/O Scheduler portions
10 Nick Piggin <piggin@cyberone.com.au>
11
12Introduction:
13
14These are some notes describing some aspects of the 2.5 block layer in the
15context of the bio rewrite. The idea is to bring out some of the key
16changes and a glimpse of the rationale behind those changes.
17
18Please mail corrections & suggestions to suparna@in.ibm.com.
19
20Credits:
21---------
22
232.5 bio rewrite:
24 Jens Axboe <axboe@suse.de>
25
26Many aspects of the generic block layer redesign were driven by and evolved
27over discussions, prior patches and the collective experience of several
28people. See sections 8 and 9 for a list of some related references.
29
30The following people helped with review comments and inputs for this
31document:
32 Christoph Hellwig <hch@infradead.org>
33 Arjan van de Ven <arjanv@redhat.com>
34 Randy Dunlap <rddunlap@osdl.org>
35 Andre Hedrick <andre@linux-ide.org>
36
37The following people helped with fixes/contributions to the bio patches
38while it was still work-in-progress:
39 David S. Miller <davem@redhat.com>
40
41
42Description of Contents:
43------------------------
44
451. Scope for tuning of logic to various needs
46 1.1 Tuning based on device or low level driver capabilities
47 - Per-queue parameters
48 - Highmem I/O support
49 - I/O scheduler modularization
50 1.2 Tuning based on high level requirements/capabilities
51 1.2.1 I/O Barriers
52 1.2.2 Request Priority/Latency
53 1.3 Direct access/bypass to lower layers for diagnostics and special
54 device operations
55 1.3.1 Pre-built commands
562. New flexible and generic but minimalist i/o structure or descriptor
57 (instead of using buffer heads at the i/o layer)
58 2.1 Requirements/Goals addressed
59 2.2 The bio struct in detail (multi-page io unit)
60 2.3 Changes in the request structure
613. Using bios
62 3.1 Setup/teardown (allocation, splitting)
63 3.2 Generic bio helper routines
64 3.2.1 Traversing segments and completion units in a request
65 3.2.2 Setting up DMA scatterlists
66 3.2.3 I/O completion
67 3.2.4 Implications for drivers that do not interpret bios (don't handle
68 multiple segments)
69 3.2.5 Request command tagging
70 3.3 I/O submission
714. The I/O scheduler
725. Scalability related changes
73 5.1 Granular locking: Removal of io_request_lock
74 5.2 Prepare for transition to 64 bit sector_t
756. Other Changes/Implications
76 6.1 Partition re-mapping handled by the generic block layer
777. A few tips on migration of older drivers
788. A list of prior/related/impacted patches/ideas
799. Other References/Discussion Threads
80
81---------------------------------------------------------------------------
82
83Bio Notes
84--------
85
86Let us discuss the changes in the context of how some overall goals for the
87block layer are addressed.
88
891. Scope for tuning the generic logic to satisfy various requirements
90
91The block layer design supports adaptable abstractions to handle common
92processing with the ability to tune the logic to an appropriate extent
93depending on the nature of the device and the requirements of the caller.
94One of the objectives of the rewrite was to increase the degree of tunability
95and to enable higher level code to utilize underlying device/driver
96capabilities to the maximum extent for better i/o performance. This is
97important especially in the light of ever improving hardware capabilities
98and application/middleware software designed to take advantage of these
99capabilities.
100
1011.1 Tuning based on low level device / driver capabilities
102
103Sophisticated devices with large built-in caches, intelligent i/o scheduling
104optimizations, high memory DMA support, etc may find some of the
105generic processing an overhead, while for less capable devices the
106generic functionality is essential for performance or correctness reasons.
107Knowledge of some of the capabilities or parameters of the device should be
108used at the generic block layer to take the right decisions on
109behalf of the driver.
110
111How is this achieved ?
112
113Tuning at a per-queue level:
114
115i. Per-queue limits/values exported to the generic layer by the driver
116
117Various parameters that the generic i/o scheduler logic uses are set at
118a per-queue level (e.g maximum request size, maximum number of segments in
119a scatter-gather list, hardsect size)
120
121Some parameters that were earlier available as global arrays indexed by
122major/minor are now directly associated with the queue. Some of these may
123move into the block device structure in the future. Some characteristics
124have been incorporated into a queue flags field rather than separate fields
125in themselves. There are blk_queue_xxx functions to set the parameters,
126rather than update the fields directly
127
128Some new queue property settings:
129
130 blk_queue_bounce_limit(q, u64 dma_address)
131 Enable I/O to highmem pages, dma_address being the
132 limit. No highmem default.
133
134 blk_queue_max_sectors(q, max_sectors)
135 Maximum size request you can handle in units of 512 byte
136 sectors. 255 default.
137
138 blk_queue_max_phys_segments(q, max_segments)
139 Maximum physical segments you can handle in a request. 128
140 default (driver limit). (See 3.2.2)
141
142 blk_queue_max_hw_segments(q, max_segments)
143 Maximum dma segments the hardware can handle in a request. 128
144 default (host adapter limit, after dma remapping).
145 (See 3.2.2)
146
147 blk_queue_max_segment_size(q, max_seg_size)
148 Maximum size of a clustered segment, 64kB default.
149
150 blk_queue_hardsect_size(q, hardsect_size)
151 Lowest possible sector size that the hardware can operate
152 on, 512 bytes default.
153
154New queue flags:
155
156 QUEUE_FLAG_CLUSTER (see 3.2.2)
157 QUEUE_FLAG_QUEUED (see 3.2.4)
158
159
160ii. High-mem i/o capabilities are now considered the default
161
162The generic bounce buffer logic, present in 2.4, where the block layer would
163by default copyin/out i/o requests on high-memory buffers to low-memory buffers
164assuming that the driver wouldn't be able to handle it directly, has been
165changed in 2.5. The bounce logic is now applied only for memory ranges
166for which the device cannot handle i/o. A driver can specify this by
167setting the queue bounce limit for the request queue for the device
168(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
169where a device is capable of handling high memory i/o.
170
171In order to enable high-memory i/o where the device is capable of supporting
172it, the pci dma mapping routines and associated data structures have now been
173modified to accomplish a direct page -> bus translation, without requiring
174a virtual address mapping (unlike the earlier scheme of virtual address
175-> bus translation). So this works uniformly for high-memory pages (which
176do not have a correponding kernel virtual address space mapping) and
177low-memory pages.
178
179Note: Please refer to DMA-mapping.txt for a discussion on PCI high mem DMA
180aspects and mapping of scatter gather lists, and support for 64 bit PCI.
181
182Special handling is required only for cases where i/o needs to happen on
183pages at physical memory addresses beyond what the device can support. In these
184cases, a bounce bio representing a buffer from the supported memory range
185is used for performing the i/o with copyin/copyout as needed depending on
186the type of the operation. For example, in case of a read operation, the
187data read has to be copied to the original buffer on i/o completion, so a
188callback routine is set up to do this, while for write, the data is copied
189from the original buffer to the bounce buffer prior to issuing the
190operation. Since an original buffer may be in a high memory area that's not
191mapped in kernel virtual addr, a kmap operation may be required for
192performing the copy, and special care may be needed in the completion path
193as it may not be in irq context. Special care is also required (by way of
194GFP flags) when allocating bounce buffers, to avoid certain highmem
195deadlock possibilities.
196
197It is also possible that a bounce buffer may be allocated from high-memory
198area that's not mapped in kernel virtual addr, but within the range that the
199device can use directly; so the bounce page may need to be kmapped during
200copy operations. [Note: This does not hold in the current implementation,
201though]
202
203There are some situations when pages from high memory may need to
204be kmapped, even if bounce buffers are not necessary. For example a device
205may need to abort DMA operations and revert to PIO for the transfer, in
206which case a virtual mapping of the page is required. For SCSI it is also
207done in some scenarios where the low level driver cannot be trusted to
208handle a single sg entry correctly. The driver is expected to perform the
209kmaps as needed on such occasions using the __bio_kmap_atomic and bio_kmap_irq
210routines as appropriate. A driver could also use the blk_queue_bounce()
211routine on its own to bounce highmem i/o to low memory for specific requests
212if so desired.
213
214iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
215
216As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
217queue or pick from (copy) existing generic schedulers and replace/override
218certain portions of it. The 2.5 rewrite provides improved modularization
219of the i/o scheduler. There are more pluggable callbacks, e.g for init,
220add request, extract request, which makes it possible to abstract specific
221i/o scheduling algorithm aspects and details outside of the generic loop.
222It also makes it possible to completely hide the implementation details of
223the i/o scheduler from block drivers.
224
225I/O scheduler wrappers are to be used instead of accessing the queue directly.
226See section 4. The I/O scheduler for details.
227
2281.2 Tuning Based on High level code capabilities
229
230i. Application capabilities for raw i/o
231
232This comes from some of the high-performance database/middleware
233requirements where an application prefers to make its own i/o scheduling
234decisions based on an understanding of the access patterns and i/o
235characteristics
236
237ii. High performance filesystems or other higher level kernel code's
238capabilities
239
240Kernel components like filesystems could also take their own i/o scheduling
241decisions for optimizing performance. Journalling filesystems may need
242some control over i/o ordering.
243
244What kind of support exists at the generic block layer for this ?
245
246The flags and rw fields in the bio structure can be used for some tuning
247from above e.g indicating that an i/o is just a readahead request, or for
248marking barrier requests (discussed next), or priority settings (currently
249unused). As far as user applications are concerned they would need an
250additional mechanism either via open flags or ioctls, or some other upper
251level mechanism to communicate such settings to block.
252
2531.2.1 I/O Barriers
254
255There is a way to enforce strict ordering for i/os through barriers.
256All requests before a barrier point must be serviced before the barrier
257request and any other requests arriving after the barrier will not be
258serviced until after the barrier has completed. This is useful for higher
259level control on write ordering, e.g flushing a log of committed updates
260to disk before the corresponding updates themselves.
261
262A flag in the bio structure, BIO_BARRIER is used to identify a barrier i/o.
263The generic i/o scheduler would make sure that it places the barrier request and
264all other requests coming after it after all the previous requests in the
265queue. Barriers may be implemented in different ways depending on the
266driver. A SCSI driver for example could make use of ordered tags to
267preserve the necessary ordering with a lower impact on throughput. For IDE
268this might be two sync cache flush: a pre and post flush when encountering
269a barrier write.
270
271There is a provision for queues to indicate what kind of barriers they
272can provide. This is as of yet unmerged, details will be added here once it
273is in the kernel.
274
2751.2.2 Request Priority/Latency
276
277Todo/Under discussion:
278Arjan's proposed request priority scheme allows higher levels some broad
279 control (high/med/low) over the priority of an i/o request vs other pending
280 requests in the queue. For example it allows reads for bringing in an
281 executable page on demand to be given a higher priority over pending write
282 requests which haven't aged too much on the queue. Potentially this priority
283 could even be exposed to applications in some manner, providing higher level
284 tunability. Time based aging avoids starvation of lower priority
285 requests. Some bits in the bi_rw flags field in the bio structure are
286 intended to be used for this priority information.
287
288
2891.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
290 (e.g Diagnostics, Systems Management)
291
292There are situations where high-level code needs to have direct access to
293the low level device capabilities or requires the ability to issue commands
294to the device bypassing some of the intermediate i/o layers.
295These could, for example, be special control commands issued through ioctl
296interfaces, or could be raw read/write commands that stress the drive's
297capabilities for certain kinds of fitness tests. Having direct interfaces at
298multiple levels without having to pass through upper layers makes
299it possible to perform bottom up validation of the i/o path, layer by
300layer, starting from the media.
301
302The normal i/o submission interfaces, e.g submit_bio, could be bypassed
303for specially crafted requests which such ioctl or diagnostics
304interfaces would typically use, and the elevator add_request routine
305can instead be used to directly insert such requests in the queue or preferably
306the blk_do_rq routine can be used to place the request on the queue and
307wait for completion. Alternatively, sometimes the caller might just
308invoke a lower level driver specific interface with the request as a
309parameter.
310
311If the request is a means for passing on special information associated with
312the command, then such information is associated with the request->special
313field (rather than misuse the request->buffer field which is meant for the
314request data buffer's virtual mapping).
315
316For passing request data, the caller must build up a bio descriptor
317representing the concerned memory buffer if the underlying driver interprets
318bio segments or uses the block layer end*request* functions for i/o
319completion. Alternatively one could directly use the request->buffer field to
320specify the virtual address of the buffer, if the driver expects buffer
321addresses passed in this way and ignores bio entries for the request type
322involved. In the latter case, the driver would modify and manage the
323request->buffer, request->sector and request->nr_sectors or
324request->current_nr_sectors fields itself rather than using the block layer
325end_request or end_that_request_first completion interfaces.
326(See 2.3 or Documentation/block/request.txt for a brief explanation of
327the request structure fields)
328
329[TBD: end_that_request_last should be usable even in this case;
330Perhaps an end_that_direct_request_first routine could be implemented to make
331handling direct requests easier for such drivers; Also for drivers that
332expect bios, a helper function could be provided for setting up a bio
333corresponding to a data buffer]
334
335<JENS: I dont understand the above, why is end_that_request_first() not
336usable? Or _last for that matter. I must be missing something>
337<SUP: What I meant here was that if the request doesn't have a bio, then
338 end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
339 and hence can't be used for advancing request state settings on the
340 completion of partial transfers. The driver has to modify these fields
341 directly by hand.
342 This is because end_that_request_first only iterates over the bio list,
343 and always returns 0 if there are none associated with the request.
344 _last works OK in this case, and is not a problem, as I mentioned earlier
345>
346
3471.3.1 Pre-built Commands
348
349A request can be created with a pre-built custom command to be sent directly
350to the device. The cmd block in the request structure has room for filling
351in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
352command pre-building, and the type of the request is now indicated
353through rq->flags instead of via rq->cmd)
354
355The request structure flags can be set up to indicate the type of request
356in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
357packet command issued via blk_do_rq, REQ_SPECIAL: special request).
358
359It can help to pre-build device commands for requests in advance.
360Drivers can now specify a request prepare function (q->prep_rq_fn) that the
361block layer would invoke to pre-build device commands for a given request,
362or perform other preparatory processing for the request. This is routine is
363called by elv_next_request(), i.e. typically just before servicing a request.
364(The prepare function would not be called for requests that have REQ_DONTPREP
365enabled)
366
367Aside:
368 Pre-building could possibly even be done early, i.e before placing the
369 request on the queue, rather than construct the command on the fly in the
370 driver while servicing the request queue when it may affect latencies in
371 interrupt context or responsiveness in general. One way to add early
372 pre-building would be to do it whenever we fail to merge on a request.
373 Now REQ_NOMERGE is set in the request flags to skip this one in the future,
374 which means that it will not change before we feed it to the device. So
375 the pre-builder hook can be invoked there.
376
377
3782. Flexible and generic but minimalist i/o structure/descriptor.
379
3802.1 Reason for a new structure and requirements addressed
381
382Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
383layer, and the low level request structure was associated with a chain of
384buffer heads for a contiguous i/o request. This led to certain inefficiencies
385when it came to large i/o requests and readv/writev style operations, as it
386forced such requests to be broken up into small chunks before being passed
387on to the generic block layer, only to be merged by the i/o scheduler
388when the underlying device was capable of handling the i/o in one shot.
389Also, using the buffer head as an i/o structure for i/os that didn't originate
390from the buffer cache unecessarily added to the weight of the descriptors
391which were generated for each such chunk.
392
393The following were some of the goals and expectations considered in the
394redesign of the block i/o data structure in 2.5.
395
396i. Should be appropriate as a descriptor for both raw and buffered i/o -
397 avoid cache related fields which are irrelevant in the direct/page i/o path,
398 or filesystem block size alignment restrictions which may not be relevant
399 for raw i/o.
400ii. Ability to represent high-memory buffers (which do not have a virtual
401 address mapping in kernel address space).
402iii.Ability to represent large i/os w/o unecessarily breaking them up (i.e
403 greater than PAGE_SIZE chunks in one shot)
404iv. At the same time, ability to retain independent identity of i/os from
405 different sources or i/o units requiring individual completion (e.g. for
406 latency reasons)
407v. Ability to represent an i/o involving multiple physical memory segments
408 (including non-page aligned page fragments, as specified via readv/writev)
409 without unecessarily breaking it up, if the underlying device is capable of
410 handling it.
411vi. Preferably should be based on a memory descriptor structure that can be
412 passed around different types of subsystems or layers, maybe even
413 networking, without duplication or extra copies of data/descriptor fields
414 themselves in the process
415vii.Ability to handle the possibility of splits/merges as the structure passes
416 through layered drivers (lvm, md, evms), with minimal overhead.
417
418The solution was to define a new structure (bio) for the block layer,
419instead of using the buffer head structure (bh) directly, the idea being
420avoidance of some associated baggage and limitations. The bio structure
421is uniformly used for all i/o at the block layer ; it forms a part of the
422bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
423mapped to bio structures.
424
4252.2 The bio struct
426
427The bio structure uses a vector representation pointing to an array of tuples
428of <page, offset, len> to describe the i/o buffer, and has various other
429fields describing i/o parameters and state that needs to be maintained for
430performing the i/o.
431
432Notice that this representation means that a bio has no virtual address
433mapping at all (unlike buffer heads).
434
435struct bio_vec {
436 struct page *bv_page;
437 unsigned short bv_len;
438 unsigned short bv_offset;
439};
440
441/*
442 * main unit of I/O for the block layer and lower layers (ie drivers)
443 */
444struct bio {
445 sector_t bi_sector;
446 struct bio *bi_next; /* request queue link */
447 struct block_device *bi_bdev; /* target device */
448 unsigned long bi_flags; /* status, command, etc */
449 unsigned long bi_rw; /* low bits: r/w, high: priority */
450
451 unsigned int bi_vcnt; /* how may bio_vec's */
452 unsigned int bi_idx; /* current index into bio_vec array */
453
454 unsigned int bi_size; /* total size in bytes */
455 unsigned short bi_phys_segments; /* segments after physaddr coalesce*/
456 unsigned short bi_hw_segments; /* segments after DMA remapping */
457 unsigned int bi_max; /* max bio_vecs we can hold
458 used as index into pool */
459 struct bio_vec *bi_io_vec; /* the actual vec list */
460 bio_end_io_t *bi_end_io; /* bi_end_io (bio) */
461 atomic_t bi_cnt; /* pin count: free when it hits zero */
462 void *bi_private;
463 bio_destructor_t *bi_destructor; /* bi_destructor (bio) */
464};
465
466With this multipage bio design:
467
468- Large i/os can be sent down in one go using a bio_vec list consisting
469 of an array of <page, offset, len> fragments (similar to the way fragments
470 are represented in the zero-copy network code)
471- Splitting of an i/o request across multiple devices (as in the case of
472 lvm or raid) is achieved by cloning the bio (where the clone points to
473 the same bi_io_vec array, but with the index and size accordingly modified)
474- A linked list of bios is used as before for unrelated merges (*) - this
475 avoids reallocs and makes independent completions easier to handle.
476- Code that traverses the req list needs to make a distinction between
477 segments of a request (bio_for_each_segment) and the distinct completion
478 units/bios (rq_for_each_bio).
479- Drivers which can't process a large bio in one shot can use the bi_idx
480 field to keep track of the next bio_vec entry to process.
481 (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
482 [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
483 bi_offset an len fields]
484
485(*) unrelated merges -- a request ends up containing two or more bios that
486 didn't originate from the same place.
487
488bi_end_io() i/o callback gets called on i/o completion of the entire bio.
489
490At a lower level, drivers build a scatter gather list from the merged bios.
491The scatter gather list is in the form of an array of <page, offset, len>
492entries with their corresponding dma address mappings filled in at the
493appropriate time. As an optimization, contiguous physical pages can be
494covered by a single entry where <page> refers to the first page and <len>
495covers the range of pages (upto 16 contiguous pages could be covered this
496way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
497the sg list.
498
499Note: Right now the only user of bios with more than one page is ll_rw_kio,
500which in turn means that only raw I/O uses it (direct i/o may not work
501right now). The intent however is to enable clustering of pages etc to
502become possible. The pagebuf abstraction layer from SGI also uses multi-page
503bios, but that is currently not included in the stock development kernels.
504The same is true of Andrew Morton's work-in-progress multipage bio writeout
505and readahead patches.
506
5072.3 Changes in the Request Structure
508
509The request structure is the structure that gets passed down to low level
510drivers. The block layer make_request function builds up a request structure,
511places it on the queue and invokes the drivers request_fn. The driver makes
512use of block layer helper routine elv_next_request to pull the next request
513off the queue. Control or diagnostic functions might bypass block and directly
514invoke underlying driver entry points passing in a specially constructed
515request structure.
516
517Only some relevant fields (mainly those which changed or may be referred
518to in some of the discussion here) are listed below, not necessarily in
519the order in which they occur in the structure (see include/linux/blkdev.h)
520Refer to Documentation/block/request.txt for details about all the request
521structure fields and a quick reference about the layers which are
522supposed to use or modify those fields.
523
524struct request {
525 struct list_head queuelist; /* Not meant to be directly accessed by
526 the driver.
527 Used by q->elv_next_request_fn
528 rq->queue is gone
529 */
530 .
531 .
532 unsigned char cmd[16]; /* prebuilt command data block */
533 unsigned long flags; /* also includes earlier rq->cmd settings */
534 .
535 .
536 sector_t sector; /* this field is now of type sector_t instead of int
537 preparation for 64 bit sectors */
538 .
539 .
540
541 /* Number of scatter-gather DMA addr+len pairs after
542 * physical address coalescing is performed.
543 */
544 unsigned short nr_phys_segments;
545
546 /* Number of scatter-gather addr+len pairs after
547 * physical and DMA remapping hardware coalescing is performed.
548 * This is the number of scatter-gather entries the driver
549 * will actually have to deal with after DMA mapping is done.
550 */
551 unsigned short nr_hw_segments;
552
553 /* Various sector counts */
554 unsigned long nr_sectors; /* no. of sectors left: driver modifiable */
555 unsigned long hard_nr_sectors; /* block internal copy of above */
556 unsigned int current_nr_sectors; /* no. of sectors left in the
557 current segment:driver modifiable */
558 unsigned long hard_cur_sectors; /* block internal copy of the above */
559 .
560 .
561 int tag; /* command tag associated with request */
562 void *special; /* same as before */
563 char *buffer; /* valid only for low memory buffers upto
564 current_nr_sectors */
565 .
566 .
567 struct bio *bio, *biotail; /* bio list instead of bh */
568 struct request_list *rl;
569}
570
571See the rq_flag_bits definitions for an explanation of the various flags
572available. Some bits are used by the block layer or i/o scheduler.
573
574The behaviour of the various sector counts are almost the same as before,
575except that since we have multi-segment bios, current_nr_sectors refers
576to the numbers of sectors in the current segment being processed which could
577be one of the many segments in the current bio (i.e i/o completion unit).
578The nr_sectors value refers to the total number of sectors in the whole
579request that remain to be transferred (no change). The purpose of the
580hard_xxx values is for block to remember these counts every time it hands
581over the request to the driver. These values are updated by block on
582end_that_request_first, i.e. every time the driver completes a part of the
583transfer and invokes block end*request helpers to mark this. The
584driver should not modify these values. The block layer sets up the
585nr_sectors and current_nr_sectors fields (based on the corresponding
586hard_xxx values and the number of bytes transferred) and updates it on
587every transfer that invokes end_that_request_first. It does the same for the
588buffer, bio, bio->bi_idx fields too.
589
590The buffer field is just a virtual address mapping of the current segment
591of the i/o buffer in cases where the buffer resides in low-memory. For high
592memory i/o, this field is not valid and must not be used by drivers.
593
594Code that sets up its own request structures and passes them down to
595a driver needs to be careful about interoperation with the block layer helper
596functions which the driver uses. (Section 1.3)
597
5983. Using bios
599
6003.1 Setup/Teardown
601
602There are routines for managing the allocation, and reference counting, and
603freeing of bios (bio_alloc, bio_get, bio_put).
604
605This makes use of Ingo Molnar's mempool implementation, which enables
606subsystems like bio to maintain their own reserve memory pools for guaranteed
607deadlock-free allocations during extreme VM load. For example, the VM
608subsystem makes use of the block layer to writeout dirty pages in order to be
609able to free up memory space, a case which needs careful handling. The
610allocation logic draws from the preallocated emergency reserve in situations
611where it cannot allocate through normal means. If the pool is empty and it
612can wait, then it would trigger action that would help free up memory or
613replenish the pool (without deadlocking) and wait for availability in the pool.
614If it is in IRQ context, and hence not in a position to do this, allocation
615could fail if the pool is empty. In general mempool always first tries to
616perform allocation without having to wait, even if it means digging into the
617pool as long it is not less that 50% full.
618
619On a free, memory is released to the pool or directly freed depending on
620the current availability in the pool. The mempool interface lets the
621subsystem specify the routines to be used for normal alloc and free. In the
622case of bio, these routines make use of the standard slab allocator.
623
624The caller of bio_alloc is expected to taken certain steps to avoid
625deadlocks, e.g. avoid trying to allocate more memory from the pool while
626already holding memory obtained from the pool.
627[TBD: This is a potential issue, though a rare possibility
628 in the bounce bio allocation that happens in the current code, since
629 it ends up allocating a second bio from the same pool while
630 holding the original bio ]
631
632Memory allocated from the pool should be released back within a limited
633amount of time (in the case of bio, that would be after the i/o is completed).
634This ensures that if part of the pool has been used up, some work (in this
635case i/o) must already be in progress and memory would be available when it
636is over. If allocating from multiple pools in the same code path, the order
637or hierarchy of allocation needs to be consistent, just the way one deals
638with multiple locks.
639
640The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
641for a non-clone bio. There are the 6 pools setup for different size biovecs,
642so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
643given size from these slabs.
644
645The bi_destructor() routine takes into account the possibility of the bio
646having originated from a different source (see later discussions on
647n/w to block transfers and kvec_cb)
648
649The bio_get() routine may be used to hold an extra reference on a bio prior
650to i/o submission, if the bio fields are likely to be accessed after the
651i/o is issued (since the bio may otherwise get freed in case i/o completion
652happens in the meantime).
653
654The bio_clone() routine may be used to duplicate a bio, where the clone
655shares the bio_vec_list with the original bio (i.e. both point to the
656same bio_vec_list). This would typically be used for splitting i/o requests
657in lvm or md.
658
6593.2 Generic bio helper Routines
660
6613.2.1 Traversing segments and completion units in a request
662
663The macros bio_for_each_segment() and rq_for_each_bio() should be used for
664traversing the bios in the request list (drivers should avoid directly
665trying to do it themselves). Using these helpers should also make it easier
666to cope with block changes in the future.
667
668 rq_for_each_bio(bio, rq)
669 bio_for_each_segment(bio_vec, bio, i)
670 /* bio_vec is now current segment */
671
672I/O completion callbacks are per-bio rather than per-segment, so drivers
673that traverse bio chains on completion need to keep that in mind. Drivers
674which don't make a distinction between segments and completion units would
675need to be reorganized to support multi-segment bios.
676
6773.2.2 Setting up DMA scatterlists
678
679The blk_rq_map_sg() helper routine would be used for setting up scatter
680gather lists from a request, so a driver need not do it on its own.
681
682 nr_segments = blk_rq_map_sg(q, rq, scatterlist);
683
684The helper routine provides a level of abstraction which makes it easier
685to modify the internals of request to scatterlist conversion down the line
686without breaking drivers. The blk_rq_map_sg routine takes care of several
687things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
688is set) and correct segment accounting to avoid exceeding the limits which
689the i/o hardware can handle, based on various queue properties.
690
691- Prevents a clustered segment from crossing a 4GB mem boundary
692- Avoids building segments that would exceed the number of physical
693 memory segments that the driver can handle (phys_segments) and the
694 number that the underlying hardware can handle at once, accounting for
695 DMA remapping (hw_segments) (i.e. IOMMU aware limits).
696
697Routines which the low level driver can use to set up the segment limits:
698
699blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
700hw data segments in a request (i.e. the maximum number of address/length
701pairs the host adapter can actually hand to the device at once)
702
703blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
704of physical data segments in a request (i.e. the largest sized scatter list
705a driver could handle)
706
7073.2.3 I/O completion
708
709The existing generic block layer helper routines end_request,
710end_that_request_first and end_that_request_last can be used for i/o
711completion (and setting things up so the rest of the i/o or the next
712request can be kicked of) as before. With the introduction of multi-page
713bio support, end_that_request_first requires an additional argument indicating
714the number of sectors completed.
715
7163.2.4 Implications for drivers that do not interpret bios (don't handle
717 multiple segments)
718
719Drivers that do not interpret bios e.g those which do not handle multiple
720segments and do not support i/o into high memory addresses (require bounce
721buffers) and expect only virtually mapped buffers, can access the rq->buffer
722field. As before the driver should use current_nr_sectors to determine the
723size of remaining data in the current segment (that is the maximum it can
724transfer in one go unless it interprets segments), and rely on the block layer
725end_request, or end_that_request_first/last to take care of all accounting
726and transparent mapping of the next bio segment when a segment boundary
727is crossed on completion of a transfer. (The end*request* functions should
728be used if only if the request has come down from block/bio path, not for
729direct access requests which only specify rq->buffer without a valid rq->bio)
730
7313.2.5 Generic request command tagging
732
7333.2.5.1 Tag helpers
734
735Block now offers some simple generic functionality to help support command
736queueing (typically known as tagged command queueing), ie manage more than
737one outstanding command on a queue at any given time.
738
739 blk_queue_init_tags(request_queue_t *q, int depth)
740
741 Initialize internal command tagging structures for a maximum
742 depth of 'depth'.
743
744 blk_queue_free_tags((request_queue_t *q)
745
746 Teardown tag info associated with the queue. This will be done
747 automatically by block if blk_queue_cleanup() is called on a queue
748 that is using tagging.
749
750The above are initialization and exit management, the main helpers during
751normal operations are:
752
753 blk_queue_start_tag(request_queue_t *q, struct request *rq)
754
755 Start tagged operation for this request. A free tag number between
756 0 and 'depth' is assigned to the request (rq->tag holds this number),
757 and 'rq' is added to the internal tag management. If the maximum depth
758 for this queue is already achieved (or if the tag wasn't started for
759 some other reason), 1 is returned. Otherwise 0 is returned.
760
761 blk_queue_end_tag(request_queue_t *q, struct request *rq)
762
763 End tagged operation on this request. 'rq' is removed from the internal
764 book keeping structures.
765
766To minimize struct request and queue overhead, the tag helpers utilize some
767of the same request members that are used for normal request queue management.
768This means that a request cannot both be an active tag and be on the queue
769list at the same time. blk_queue_start_tag() will remove the request, but
770the driver must remember to call blk_queue_end_tag() before signalling
771completion of the request to the block layer. This means ending tag
772operations before calling end_that_request_last()! For an example of a user
773of these helpers, see the IDE tagged command queueing support.
774
775Certain hardware conditions may dictate a need to invalidate the block tag
776queue. For instance, on IDE any tagged request error needs to clear both
777the hardware and software block queue and enable the driver to sanely restart
778all the outstanding requests. There's a third helper to do that:
779
780 blk_queue_invalidate_tags(request_queue_t *q)
781
782 Clear the internal block tag queue and readd all the pending requests
783 to the request queue. The driver will receive them again on the
784 next request_fn run, just like it did the first time it encountered
785 them.
786
7873.2.5.2 Tag info
788
789Some block functions exist to query current tag status or to go from a
790tag number to the associated request. These are, in no particular order:
791
792 blk_queue_tagged(q)
793
794 Returns 1 if the queue 'q' is using tagging, 0 if not.
795
796 blk_queue_tag_request(q, tag)
797
798 Returns a pointer to the request associated with tag 'tag'.
799
800 blk_queue_tag_depth(q)
801
802 Return current queue depth.
803
804 blk_queue_tag_queue(q)
805
806 Returns 1 if the queue can accept a new queued command, 0 if we are
807 at the maximum depth already.
808
809 blk_queue_rq_tagged(rq)
810
811 Returns 1 if the request 'rq' is tagged.
812
8133.2.5.2 Internal structure
814
815Internally, block manages tags in the blk_queue_tag structure:
816
817 struct blk_queue_tag {
818 struct request **tag_index; /* array or pointers to rq */
819 unsigned long *tag_map; /* bitmap of free tags */
820 struct list_head busy_list; /* fifo list of busy tags */
821 int busy; /* queue depth */
822 int max_depth; /* max queue depth */
823 };
824
825Most of the above is simple and straight forward, however busy_list may need
826a bit of explaining. Normally we don't care too much about request ordering,
827but in the event of any barrier requests in the tag queue we need to ensure
828that requests are restarted in the order they were queue. This may happen
829if the driver needs to use blk_queue_invalidate_tags().
830
831Tagging also defines a new request flag, REQ_QUEUED. This is set whenever
832a request is currently tagged. You should not use this flag directly,
833blk_rq_tagged(rq) is the portable way to do so.
834
8353.3 I/O Submission
836
837The routine submit_bio() is used to submit a single io. Higher level i/o
838routines make use of this:
839
840(a) Buffered i/o:
841The routine submit_bh() invokes submit_bio() on a bio corresponding to the
842bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
843
844(b) Kiobuf i/o (for raw/direct i/o):
845The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
846maps the array to one or more multi-page bios, issuing submit_bio() to
847perform the i/o on each of these.
848
849The embedded bh array in the kiobuf structure has been removed and no
850preallocation of bios is done for kiobufs. [The intent is to remove the
851blocks array as well, but it's currently in there to kludge around direct i/o.]
852Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
853
854Todo/Observation:
855
856 A single kiobuf structure is assumed to correspond to a contiguous range
857 of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
858 So right now it wouldn't work for direct i/o on non-contiguous blocks.
859 This is to be resolved. The eventual direction is to replace kiobuf
860 by kvec's.
861
862 Badari Pulavarty has a patch to implement direct i/o correctly using
863 bio and kvec.
864
865
866(c) Page i/o:
867Todo/Under discussion:
868
869 Andrew Morton's multi-page bio patches attempt to issue multi-page
870 writeouts (and reads) from the page cache, by directly building up
871 large bios for submission completely bypassing the usage of buffer
872 heads. This work is still in progress.
873
874 Christoph Hellwig had some code that uses bios for page-io (rather than
875 bh). This isn't included in bio as yet. Christoph was also working on a
876 design for representing virtual/real extents as an entity and modifying
877 some of the address space ops interfaces to utilize this abstraction rather
878 than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
879 abstraction, but intended to be as lightweight as possible).
880
881(d) Direct access i/o:
882Direct access requests that do not contain bios would be submitted differently
883as discussed earlier in section 1.3.
884
885Aside:
886
887 Kvec i/o:
888
889 Ben LaHaise's aio code uses a slighly different structure instead
890 of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
891 tuples (very much like the networking code), together with a callback function
892 and data pointer. This is embedded into a brw_cb structure when passed
893 to brw_kvec_async().
894
895 Now it should be possible to directly map these kvecs to a bio. Just as while
896 cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
897 array pointer to point to the veclet array in kvecs.
898
899 TBD: In order for this to work, some changes are needed in the way multi-page
900 bios are handled today. The values of the tuples in such a vector passed in
901 from higher level code should not be modified by the block layer in the course
902 of its request processing, since that would make it hard for the higher layer
903 to continue to use the vector descriptor (kvec) after i/o completes. Instead,
904 all such transient state should either be maintained in the request structure,
905 and passed on in some way to the endio completion routine.
906
907
9084. The I/O scheduler
Tejun Heo4c9f7832005-10-20 16:47:40 +0200909I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch
910queue and specific I/O schedulers. Unless stated otherwise, elevator is used
911to refer to both parts and I/O scheduler to specific I/O schedulers.
912
913Block layer implements generic dispatch queue in ll_rw_blk.c and elevator.c.
914The generic dispatch queue is responsible for properly ordering barrier
915requests, requeueing, handling non-fs requests and all other subtleties.
916
917Specific I/O schedulers are responsible for ordering normal filesystem
918requests. They can also choose to delay certain requests to improve
919throughput or whatever purpose. As the plural form indicates, there are
920multiple I/O schedulers. They can be built as modules but at least one should
921be built inside the kernel. Each queue can choose different one and can also
922change to another one dynamically.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700923
924A block layer call to the i/o scheduler follows the convention elv_xxx(). This
925calls elevator_xxx_fn in the elevator switch (drivers/block/elevator.c). Oh,
926xxx and xxx might not match exactly, but use your imagination. If an elevator
927doesn't implement a function, the switch does nothing or some minimal house
928keeping work.
929
9304.1. I/O scheduler API
931
932The functions an elevator may implement are: (* are mandatory)
933elevator_merge_fn called to query requests for merge with a bio
934
Tejun Heo4c9f7832005-10-20 16:47:40 +0200935elevator_merge_req_fn called when two requests get merged. the one
936 which gets merged into the other one will be
937 never seen by I/O scheduler again. IOW, after
938 being merged, the request is gone.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700939
940elevator_merged_fn called when a request in the scheduler has been
941 involved in a merge. It is used in the deadline
942 scheduler for example, to reposition the request
943 if its sorting order has changed.
944
Tejun Heo4c9f7832005-10-20 16:47:40 +0200945elevator_dispatch_fn fills the dispatch queue with ready requests.
946 I/O schedulers are free to postpone requests by
947 not filling the dispatch queue unless @force
948 is non-zero. Once dispatched, I/O schedulers
949 are not allowed to manipulate the requests -
950 they belong to generic dispatch queue.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700951
Tejun Heo4c9f7832005-10-20 16:47:40 +0200952elevator_add_req_fn called to add a new request into the scheduler
Linus Torvalds1da177e2005-04-16 15:20:36 -0700953
954elevator_queue_empty_fn returns true if the merge queue is empty.
955 Drivers shouldn't use this, but rather check
956 if elv_next_request is NULL (without losing the
957 request if one exists!)
958
Linus Torvalds1da177e2005-04-16 15:20:36 -0700959elevator_former_req_fn
960elevator_latter_req_fn These return the request before or after the
961 one specified in disk sort order. Used by the
962 block layer to find merge possibilities.
963
Tejun Heo4c9f7832005-10-20 16:47:40 +0200964elevator_completed_req_fn called when a request is completed.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700965
966elevator_may_queue_fn returns true if the scheduler wants to allow the
967 current context to queue a new request even if
968 it is over the queue limit. This must be used
969 very carefully!!
970
971elevator_set_req_fn
972elevator_put_req_fn Must be used to allocate and free any elevator
Tejun Heo4c9f7832005-10-20 16:47:40 +0200973 specific storage for a request.
974
975elevator_activate_req_fn Called when device driver first sees a request.
976 I/O schedulers can use this callback to
977 determine when actual execution of a request
978 starts.
979elevator_deactivate_req_fn Called when device driver decides to delay
980 a request by requeueing it.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700981
982elevator_init_fn
983elevator_exit_fn Allocate and free any elevator specific storage
984 for a queue.
985
Tejun Heo4c9f7832005-10-20 16:47:40 +02009864.2 Request flows seen by I/O schedulers
987All requests seens by I/O schedulers strictly follow one of the following three
988flows.
989
990 set_req_fn ->
991
992 i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
993 (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
994 ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn
995 iii. [none]
996
997 -> put_req_fn
998
9994.3 I/O scheduler implementation
Linus Torvalds1da177e2005-04-16 15:20:36 -07001000The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
1001optimal disk scan and request servicing performance (based on generic
1002principles and device capabilities), optimized for:
1003i. improved throughput
1004ii. improved latency
1005iii. better utilization of h/w & CPU time
1006
1007Characteristics:
1008
1009i. Binary tree
1010AS and deadline i/o schedulers use red black binary trees for disk position
1011sorting and searching, and a fifo linked list for time-based searching. This
1012gives good scalability and good availablility of information. Requests are
1013almost always dispatched in disk sort order, so a cache is kept of the next
1014request in sort order to prevent binary tree lookups.
1015
1016This arrangement is not a generic block layer characteristic however, so
1017elevators may implement queues as they please.
1018
Tejun Heo4c9f7832005-10-20 16:47:40 +02001019ii. Merge hash
Linus Torvalds1da177e2005-04-16 15:20:36 -07001020AS and deadline use a hash table indexed by the last sector of a request. This
1021enables merging code to quickly look up "back merge" candidates, even when
1022multiple I/O streams are being performed at once on one disk.
1023
1024"Front merges", a new request being merged at the front of an existing request,
1025are far less common than "back merges" due to the nature of most I/O patterns.
1026Front merges are handled by the binary trees in AS and deadline schedulers.
1027
Tejun Heo4c9f7832005-10-20 16:47:40 +02001028iii. Plugging the queue to batch requests in anticipation of opportunities for
1029 merge/sort optimizations
Linus Torvalds1da177e2005-04-16 15:20:36 -07001030
1031This is just the same as in 2.4 so far, though per-device unplugging
1032support is anticipated for 2.5. Also with a priority-based i/o scheduler,
1033such decisions could be based on request priorities.
1034
1035Plugging is an approach that the current i/o scheduling algorithm resorts to so
1036that it collects up enough requests in the queue to be able to take
1037advantage of the sorting/merging logic in the elevator. If the
1038queue is empty when a request comes in, then it plugs the request queue
1039(sort of like plugging the bottom of a vessel to get fluid to build up)
1040till it fills up with a few more requests, before starting to service
1041the requests. This provides an opportunity to merge/sort the requests before
1042passing them down to the device. There are various conditions when the queue is
1043unplugged (to open up the flow again), either through a scheduled task or
1044could be on demand. For example wait_on_buffer sets the unplugging going
1045(by running tq_disk) so the read gets satisfied soon. So in the read case,
1046the queue gets explicitly unplugged as part of waiting for completion,
1047in fact all queues get unplugged as a side-effect.
1048
1049Aside:
1050 This is kind of controversial territory, as it's not clear if plugging is
1051 always the right thing to do. Devices typically have their own queues,
1052 and allowing a big queue to build up in software, while letting the device be
1053 idle for a while may not always make sense. The trick is to handle the fine
1054 balance between when to plug and when to open up. Also now that we have
1055 multi-page bios being queued in one shot, we may not need to wait to merge
1056 a big request from the broken up pieces coming by.
1057
1058 Per-queue granularity unplugging (still a Todo) may help reduce some of the
1059 concerns with just a single tq_disk flush approach. Something like
1060 blk_kick_queue() to unplug a specific queue (right away ?)
1061 or optionally, all queues, is in the plan.
1062
Tejun Heo4c9f7832005-10-20 16:47:40 +020010634.4 I/O contexts
Linus Torvalds1da177e2005-04-16 15:20:36 -07001064I/O contexts provide a dynamically allocated per process data area. They may
1065be used in I/O schedulers, and in the block layer (could be used for IO statis,
Ben Collins1d193f42005-11-15 00:09:21 -08001066priorities for example). See *io_context in block/ll_rw_blk.c, and as-iosched.c
1067for an example of usage in an i/o scheduler.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001068
1069
10705. Scalability related changes
1071
10725.1 Granular Locking: io_request_lock replaced by a per-queue lock
1073
1074The global io_request_lock has been removed as of 2.5, to avoid
1075the scalability bottleneck it was causing, and has been replaced by more
1076granular locking. The request queue structure has a pointer to the
1077lock to be used for that queue. As a result, locking can now be
1078per-queue, with a provision for sharing a lock across queues if
1079necessary (e.g the scsi layer sets the queue lock pointers to the
1080corresponding adapter lock, which results in a per host locking
1081granularity). The locking semantics are the same, i.e. locking is
1082still imposed by the block layer, grabbing the lock before
1083request_fn execution which it means that lots of older drivers
1084should still be SMP safe. Drivers are free to drop the queue
1085lock themselves, if required. Drivers that explicitly used the
1086io_request_lock for serialization need to be modified accordingly.
1087Usually it's as easy as adding a global lock:
1088
1089 static spinlock_t my_driver_lock = SPIN_LOCK_UNLOCKED;
1090
1091and passing the address to that lock to blk_init_queue().
1092
10935.2 64 bit sector numbers (sector_t prepares for 64 bit support)
1094
1095The sector number used in the bio structure has been changed to sector_t,
1096which could be defined as 64 bit in preparation for 64 bit sector support.
1097
10986. Other Changes/Implications
1099
11006.1 Partition re-mapping handled by the generic block layer
1101
1102In 2.5 some of the gendisk/partition related code has been reorganized.
1103Now the generic block layer performs partition-remapping early and thus
1104provides drivers with a sector number relative to whole device, rather than
1105having to take partition number into account in order to arrive at the true
1106sector number. The routine blk_partition_remap() is invoked by
1107generic_make_request even before invoking the queue specific make_request_fn,
1108so the i/o scheduler also gets to operate on whole disk sector numbers. This
1109should typically not require changes to block drivers, it just never gets
1110to invoke its own partition sector offset calculations since all bios
1111sent are offset from the beginning of the device.
1112
1113
11147. A Few Tips on Migration of older drivers
1115
1116Old-style drivers that just use CURRENT and ignores clustered requests,
1117may not need much change. The generic layer will automatically handle
1118clustered requests, multi-page bios, etc for the driver.
1119
1120For a low performance driver or hardware that is PIO driven or just doesn't
1121support scatter-gather changes should be minimal too.
1122
1123The following are some points to keep in mind when converting old drivers
1124to bio.
1125
1126Drivers should use elv_next_request to pick up requests and are no longer
1127supposed to handle looping directly over the request list.
1128(struct request->queue has been removed)
1129
1130Now end_that_request_first takes an additional number_of_sectors argument.
1131It used to handle always just the first buffer_head in a request, now
1132it will loop and handle as many sectors (on a bio-segment granularity)
1133as specified.
1134
1135Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
1136right thing to use is bio_endio(bio, uptodate) instead.
1137
1138If the driver is dropping the io_request_lock from its request_fn strategy,
1139then it just needs to replace that with q->queue_lock instead.
1140
1141As described in Sec 1.1, drivers can set max sector size, max segment size
1142etc per queue now. Drivers that used to define their own merge functions i
1143to handle things like this can now just use the blk_queue_* functions at
1144blk_init_queue time.
1145
1146Drivers no longer have to map a {partition, sector offset} into the
1147correct absolute location anymore, this is done by the block layer, so
1148where a driver received a request ala this before:
1149
1150 rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */
1151 rq->sector = 0; /* first sector on hda5 */
1152
1153 it will now see
1154
1155 rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */
1156 rq->sector = 123128; /* offset from start of disk */
1157
1158As mentioned, there is no virtual mapping of a bio. For DMA, this is
1159not a problem as the driver probably never will need a virtual mapping.
1160Instead it needs a bus mapping (pci_map_page for a single segment or
1161use blk_rq_map_sg for scatter gather) to be able to ship it to the driver. For
1162PIO drivers (or drivers that need to revert to PIO transfer once in a
1163while (IDE for example)), where the CPU is doing the actual data
1164transfer a virtual mapping is needed. If the driver supports highmem I/O,
1165(Sec 1.1, (ii) ) it needs to use __bio_kmap_atomic and bio_kmap_irq to
1166temporarily map a bio into the virtual address space.
1167
1168
11698. Prior/Related/Impacted patches
1170
11718.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
1172- orig kiobuf & raw i/o patches (now in 2.4 tree)
1173- direct kiobuf based i/o to devices (no intermediate bh's)
1174- page i/o using kiobuf
1175- kiobuf splitting for lvm (mkp)
1176- elevator support for kiobuf request merging (axboe)
11778.2. Zero-copy networking (Dave Miller)
11788.3. SGI XFS - pagebuf patches - use of kiobufs
11798.4. Multi-page pioent patch for bio (Christoph Hellwig)
11808.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
11818.6. Async i/o implementation patch (Ben LaHaise)
11828.7. EVMS layering design (IBM EVMS team)
11838.8. Larger page cache size patch (Ben LaHaise) and
1184 Large page size (Daniel Phillips)
1185 => larger contiguous physical memory buffers
11868.9. VM reservations patch (Ben LaHaise)
11878.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
11888.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
11898.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar,
1190 Badari)
11918.13 Priority based i/o scheduler - prepatches (Arjan van de Ven)
11928.14 IDE Taskfile i/o patch (Andre Hedrick)
11938.15 Multi-page writeout and readahead patches (Andrew Morton)
11948.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
1195
11969. Other References:
1197
11989.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml,
1199and Linus' comments - Jan 2001)
12009.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan
1201et al - Feb-March 2001 (many of the initial thoughts that led to bio were
1202brought up in this discusion thread)
12039.3 Discussions on mempool on lkml - Dec 2001.
1204