| Notes on the Generic Block Layer Rewrite in Linux 2.5 |
| ===================================================== |
| |
| Notes Written on Jan 15, 2002: |
| Jens Axboe <jens.axboe@oracle.com> |
| Suparna Bhattacharya <suparna@in.ibm.com> |
| |
| Last Updated May 2, 2002 |
| September 2003: Updated I/O Scheduler portions |
| Nick Piggin <npiggin@kernel.dk> |
| |
| Introduction: |
| |
| These are some notes describing some aspects of the 2.5 block layer in the |
| context of the bio rewrite. The idea is to bring out some of the key |
| changes and a glimpse of the rationale behind those changes. |
| |
| Please mail corrections & suggestions to suparna@in.ibm.com. |
| |
| Credits: |
| --------- |
| |
| 2.5 bio rewrite: |
| Jens Axboe <jens.axboe@oracle.com> |
| |
| Many aspects of the generic block layer redesign were driven by and evolved |
| over discussions, prior patches and the collective experience of several |
| people. See sections 8 and 9 for a list of some related references. |
| |
| The following people helped with review comments and inputs for this |
| document: |
| Christoph Hellwig <hch@infradead.org> |
| Arjan van de Ven <arjanv@redhat.com> |
| Randy Dunlap <rdunlap@xenotime.net> |
| Andre Hedrick <andre@linux-ide.org> |
| |
| The following people helped with fixes/contributions to the bio patches |
| while it was still work-in-progress: |
| David S. Miller <davem@redhat.com> |
| |
| |
| Description of Contents: |
| ------------------------ |
| |
| 1. Scope for tuning of logic to various needs |
| 1.1 Tuning based on device or low level driver capabilities |
| - Per-queue parameters |
| - Highmem I/O support |
| - I/O scheduler modularization |
| 1.2 Tuning based on high level requirements/capabilities |
| 1.2.1 I/O Barriers |
| 1.2.2 Request Priority/Latency |
| 1.3 Direct access/bypass to lower layers for diagnostics and special |
| device operations |
| 1.3.1 Pre-built commands |
| 2. New flexible and generic but minimalist i/o structure or descriptor |
| (instead of using buffer heads at the i/o layer) |
| 2.1 Requirements/Goals addressed |
| 2.2 The bio struct in detail (multi-page io unit) |
| 2.3 Changes in the request structure |
| 3. Using bios |
| 3.1 Setup/teardown (allocation, splitting) |
| 3.2 Generic bio helper routines |
| 3.2.1 Traversing segments and completion units in a request |
| 3.2.2 Setting up DMA scatterlists |
| 3.2.3 I/O completion |
| 3.2.4 Implications for drivers that do not interpret bios (don't handle |
| multiple segments) |
| 3.2.5 Request command tagging |
| 3.3 I/O submission |
| 4. The I/O scheduler |
| 5. Scalability related changes |
| 5.1 Granular locking: Removal of io_request_lock |
| 5.2 Prepare for transition to 64 bit sector_t |
| 6. Other Changes/Implications |
| 6.1 Partition re-mapping handled by the generic block layer |
| 7. A few tips on migration of older drivers |
| 8. A list of prior/related/impacted patches/ideas |
| 9. Other References/Discussion Threads |
| |
| --------------------------------------------------------------------------- |
| |
| Bio Notes |
| -------- |
| |
| Let us discuss the changes in the context of how some overall goals for the |
| block layer are addressed. |
| |
| 1. Scope for tuning the generic logic to satisfy various requirements |
| |
| The block layer design supports adaptable abstractions to handle common |
| processing with the ability to tune the logic to an appropriate extent |
| depending on the nature of the device and the requirements of the caller. |
| One of the objectives of the rewrite was to increase the degree of tunability |
| and to enable higher level code to utilize underlying device/driver |
| capabilities to the maximum extent for better i/o performance. This is |
| important especially in the light of ever improving hardware capabilities |
| and application/middleware software designed to take advantage of these |
| capabilities. |
| |
| 1.1 Tuning based on low level device / driver capabilities |
| |
| Sophisticated devices with large built-in caches, intelligent i/o scheduling |
| optimizations, high memory DMA support, etc may find some of the |
| generic processing an overhead, while for less capable devices the |
| generic functionality is essential for performance or correctness reasons. |
| Knowledge of some of the capabilities or parameters of the device should be |
| used at the generic block layer to take the right decisions on |
| behalf of the driver. |
| |
| How is this achieved ? |
| |
| Tuning at a per-queue level: |
| |
| i. Per-queue limits/values exported to the generic layer by the driver |
| |
| Various parameters that the generic i/o scheduler logic uses are set at |
| a per-queue level (e.g maximum request size, maximum number of segments in |
| a scatter-gather list, hardsect size) |
| |
| Some parameters that were earlier available as global arrays indexed by |
| major/minor are now directly associated with the queue. Some of these may |
| move into the block device structure in the future. Some characteristics |
| have been incorporated into a queue flags field rather than separate fields |
| in themselves. There are blk_queue_xxx functions to set the parameters, |
| rather than update the fields directly |
| |
| Some new queue property settings: |
| |
| blk_queue_bounce_limit(q, u64 dma_address) |
| Enable I/O to highmem pages, dma_address being the |
| limit. No highmem default. |
| |
| blk_queue_max_sectors(q, max_sectors) |
| Sets two variables that limit the size of the request. |
| |
| - The request queue's max_sectors, which is a soft size in |
| units of 512 byte sectors, and could be dynamically varied |
| by the core kernel. |
| |
| - The request queue's max_hw_sectors, which is a hard limit |
| and reflects the maximum size request a driver can handle |
| in units of 512 byte sectors. |
| |
| The default for both max_sectors and max_hw_sectors is |
| 255. The upper limit of max_sectors is 1024. |
| |
| blk_queue_max_phys_segments(q, max_segments) |
| Maximum physical segments you can handle in a request. 128 |
| default (driver limit). (See 3.2.2) |
| |
| blk_queue_max_hw_segments(q, max_segments) |
| Maximum dma segments the hardware can handle in a request. 128 |
| default (host adapter limit, after dma remapping). |
| (See 3.2.2) |
| |
| blk_queue_max_segment_size(q, max_seg_size) |
| Maximum size of a clustered segment, 64kB default. |
| |
| blk_queue_hardsect_size(q, hardsect_size) |
| Lowest possible sector size that the hardware can operate |
| on, 512 bytes default. |
| |
| New queue flags: |
| |
| QUEUE_FLAG_CLUSTER (see 3.2.2) |
| QUEUE_FLAG_QUEUED (see 3.2.4) |
| |
| |
| ii. High-mem i/o capabilities are now considered the default |
| |
| The generic bounce buffer logic, present in 2.4, where the block layer would |
| by default copyin/out i/o requests on high-memory buffers to low-memory buffers |
| assuming that the driver wouldn't be able to handle it directly, has been |
| changed in 2.5. The bounce logic is now applied only for memory ranges |
| for which the device cannot handle i/o. A driver can specify this by |
| setting the queue bounce limit for the request queue for the device |
| (blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out |
| where a device is capable of handling high memory i/o. |
| |
| In order to enable high-memory i/o where the device is capable of supporting |
| it, the pci dma mapping routines and associated data structures have now been |
| modified to accomplish a direct page -> bus translation, without requiring |
| a virtual address mapping (unlike the earlier scheme of virtual address |
| -> bus translation). So this works uniformly for high-memory pages (which |
| do not have a corresponding kernel virtual address space mapping) and |
| low-memory pages. |
| |
| Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion |
| on PCI high mem DMA aspects and mapping of scatter gather lists, and support |
| for 64 bit PCI. |
| |
| Special handling is required only for cases where i/o needs to happen on |
| pages at physical memory addresses beyond what the device can support. In these |
| cases, a bounce bio representing a buffer from the supported memory range |
| is used for performing the i/o with copyin/copyout as needed depending on |
| the type of the operation. For example, in case of a read operation, the |
| data read has to be copied to the original buffer on i/o completion, so a |
| callback routine is set up to do this, while for write, the data is copied |
| from the original buffer to the bounce buffer prior to issuing the |
| operation. Since an original buffer may be in a high memory area that's not |
| mapped in kernel virtual addr, a kmap operation may be required for |
| performing the copy, and special care may be needed in the completion path |
| as it may not be in irq context. Special care is also required (by way of |
| GFP flags) when allocating bounce buffers, to avoid certain highmem |
| deadlock possibilities. |
| |
| It is also possible that a bounce buffer may be allocated from high-memory |
| area that's not mapped in kernel virtual addr, but within the range that the |
| device can use directly; so the bounce page may need to be kmapped during |
| copy operations. [Note: This does not hold in the current implementation, |
| though] |
| |
| There are some situations when pages from high memory may need to |
| be kmapped, even if bounce buffers are not necessary. For example a device |
| may need to abort DMA operations and revert to PIO for the transfer, in |
| which case a virtual mapping of the page is required. For SCSI it is also |
| done in some scenarios where the low level driver cannot be trusted to |
| handle a single sg entry correctly. The driver is expected to perform the |
| kmaps as needed on such occasions using the __bio_kmap_atomic and bio_kmap_irq |
| routines as appropriate. A driver could also use the blk_queue_bounce() |
| routine on its own to bounce highmem i/o to low memory for specific requests |
| if so desired. |
| |
| iii. The i/o scheduler algorithm itself can be replaced/set as appropriate |
| |
| As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular |
| queue or pick from (copy) existing generic schedulers and replace/override |
| certain portions of it. The 2.5 rewrite provides improved modularization |
| of the i/o scheduler. There are more pluggable callbacks, e.g for init, |
| add request, extract request, which makes it possible to abstract specific |
| i/o scheduling algorithm aspects and details outside of the generic loop. |
| It also makes it possible to completely hide the implementation details of |
| the i/o scheduler from block drivers. |
| |
| I/O scheduler wrappers are to be used instead of accessing the queue directly. |
| See section 4. The I/O scheduler for details. |
| |
| 1.2 Tuning Based on High level code capabilities |
| |
| i. Application capabilities for raw i/o |
| |
| This comes from some of the high-performance database/middleware |
| requirements where an application prefers to make its own i/o scheduling |
| decisions based on an understanding of the access patterns and i/o |
| characteristics |
| |
| ii. High performance filesystems or other higher level kernel code's |
| capabilities |
| |
| Kernel components like filesystems could also take their own i/o scheduling |
| decisions for optimizing performance. Journalling filesystems may need |
| some control over i/o ordering. |
| |
| What kind of support exists at the generic block layer for this ? |
| |
| The flags and rw fields in the bio structure can be used for some tuning |
| from above e.g indicating that an i/o is just a readahead request, or for |
| marking barrier requests (discussed next), or priority settings (currently |
| unused). As far as user applications are concerned they would need an |
| additional mechanism either via open flags or ioctls, or some other upper |
| level mechanism to communicate such settings to block. |
| |
| 1.2.1 I/O Barriers |
| |
| There is a way to enforce strict ordering for i/os through barriers. |
| All requests before a barrier point must be serviced before the barrier |
| request and any other requests arriving after the barrier will not be |
| serviced until after the barrier has completed. This is useful for higher |
| level control on write ordering, e.g flushing a log of committed updates |
| to disk before the corresponding updates themselves. |
| |
| A flag in the bio structure, BIO_BARRIER is used to identify a barrier i/o. |
| The generic i/o scheduler would make sure that it places the barrier request and |
| all other requests coming after it after all the previous requests in the |
| queue. Barriers may be implemented in different ways depending on the |
| driver. For more details regarding I/O barriers, please read barrier.txt |
| in this directory. |
| |
| 1.2.2 Request Priority/Latency |
| |
| Todo/Under discussion: |
| Arjan's proposed request priority scheme allows higher levels some broad |
| control (high/med/low) over the priority of an i/o request vs other pending |
| requests in the queue. For example it allows reads for bringing in an |
| executable page on demand to be given a higher priority over pending write |
| requests which haven't aged too much on the queue. Potentially this priority |
| could even be exposed to applications in some manner, providing higher level |
| tunability. Time based aging avoids starvation of lower priority |
| requests. Some bits in the bi_rw flags field in the bio structure are |
| intended to be used for this priority information. |
| |
| |
| 1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode) |
| (e.g Diagnostics, Systems Management) |
| |
| There are situations where high-level code needs to have direct access to |
| the low level device capabilities or requires the ability to issue commands |
| to the device bypassing some of the intermediate i/o layers. |
| These could, for example, be special control commands issued through ioctl |
| interfaces, or could be raw read/write commands that stress the drive's |
| capabilities for certain kinds of fitness tests. Having direct interfaces at |
| multiple levels without having to pass through upper layers makes |
| it possible to perform bottom up validation of the i/o path, layer by |
| layer, starting from the media. |
| |
| The normal i/o submission interfaces, e.g submit_bio, could be bypassed |
| for specially crafted requests which such ioctl or diagnostics |
| interfaces would typically use, and the elevator add_request routine |
| can instead be used to directly insert such requests in the queue or preferably |
| the blk_do_rq routine can be used to place the request on the queue and |
| wait for completion. Alternatively, sometimes the caller might just |
| invoke a lower level driver specific interface with the request as a |
| parameter. |
| |
| If the request is a means for passing on special information associated with |
| the command, then such information is associated with the request->special |
| field (rather than misuse the request->buffer field which is meant for the |
| request data buffer's virtual mapping). |
| |
| For passing request data, the caller must build up a bio descriptor |
| representing the concerned memory buffer if the underlying driver interprets |
| bio segments or uses the block layer end*request* functions for i/o |
| completion. Alternatively one could directly use the request->buffer field to |
| specify the virtual address of the buffer, if the driver expects buffer |
| addresses passed in this way and ignores bio entries for the request type |
| involved. In the latter case, the driver would modify and manage the |
| request->buffer, request->sector and request->nr_sectors or |
| request->current_nr_sectors fields itself rather than using the block layer |
| end_request or end_that_request_first completion interfaces. |
| (See 2.3 or Documentation/block/request.txt for a brief explanation of |
| the request structure fields) |
| |
| [TBD: end_that_request_last should be usable even in this case; |
| Perhaps an end_that_direct_request_first routine could be implemented to make |
| handling direct requests easier for such drivers; Also for drivers that |
| expect bios, a helper function could be provided for setting up a bio |
| corresponding to a data buffer] |
| |
| <JENS: I dont understand the above, why is end_that_request_first() not |
| usable? Or _last for that matter. I must be missing something> |
| <SUP: What I meant here was that if the request doesn't have a bio, then |
| end_that_request_first doesn't modify nr_sectors or current_nr_sectors, |
| and hence can't be used for advancing request state settings on the |
| completion of partial transfers. The driver has to modify these fields |
| directly by hand. |
| This is because end_that_request_first only iterates over the bio list, |
| and always returns 0 if there are none associated with the request. |
| _last works OK in this case, and is not a problem, as I mentioned earlier |
| > |
| |
| 1.3.1 Pre-built Commands |
| |
| A request can be created with a pre-built custom command to be sent directly |
| to the device. The cmd block in the request structure has room for filling |
| in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for |
| command pre-building, and the type of the request is now indicated |
| through rq->flags instead of via rq->cmd) |
| |
| The request structure flags can be set up to indicate the type of request |
| in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC: |
| packet command issued via blk_do_rq, REQ_SPECIAL: special request). |
| |
| It can help to pre-build device commands for requests in advance. |
| Drivers can now specify a request prepare function (q->prep_rq_fn) that the |
| block layer would invoke to pre-build device commands for a given request, |
| or perform other preparatory processing for the request. This is routine is |
| called by elv_next_request(), i.e. typically just before servicing a request. |
| (The prepare function would not be called for requests that have REQ_DONTPREP |
| enabled) |
| |
| Aside: |
| Pre-building could possibly even be done early, i.e before placing the |
| request on the queue, rather than construct the command on the fly in the |
| driver while servicing the request queue when it may affect latencies in |
| interrupt context or responsiveness in general. One way to add early |
| pre-building would be to do it whenever we fail to merge on a request. |
| Now REQ_NOMERGE is set in the request flags to skip this one in the future, |
| which means that it will not change before we feed it to the device. So |
| the pre-builder hook can be invoked there. |
| |
| |
| 2. Flexible and generic but minimalist i/o structure/descriptor. |
| |
| 2.1 Reason for a new structure and requirements addressed |
| |
| Prior to 2.5, buffer heads were used as the unit of i/o at the generic block |
| layer, and the low level request structure was associated with a chain of |
| buffer heads for a contiguous i/o request. This led to certain inefficiencies |
| when it came to large i/o requests and readv/writev style operations, as it |
| forced such requests to be broken up into small chunks before being passed |
| on to the generic block layer, only to be merged by the i/o scheduler |
| when the underlying device was capable of handling the i/o in one shot. |
| Also, using the buffer head as an i/o structure for i/os that didn't originate |
| from the buffer cache unnecessarily added to the weight of the descriptors |
| which were generated for each such chunk. |
| |
| The following were some of the goals and expectations considered in the |
| redesign of the block i/o data structure in 2.5. |
| |
| i. Should be appropriate as a descriptor for both raw and buffered i/o - |
| avoid cache related fields which are irrelevant in the direct/page i/o path, |
| or filesystem block size alignment restrictions which may not be relevant |
| for raw i/o. |
| ii. Ability to represent high-memory buffers (which do not have a virtual |
| address mapping in kernel address space). |
| iii.Ability to represent large i/os w/o unnecessarily breaking them up (i.e |
| greater than PAGE_SIZE chunks in one shot) |
| iv. At the same time, ability to retain independent identity of i/os from |
| different sources or i/o units requiring individual completion (e.g. for |
| latency reasons) |
| v. Ability to represent an i/o involving multiple physical memory segments |
| (including non-page aligned page fragments, as specified via readv/writev) |
| without unnecessarily breaking it up, if the underlying device is capable of |
| handling it. |
| vi. Preferably should be based on a memory descriptor structure that can be |
| passed around different types of subsystems or layers, maybe even |
| networking, without duplication or extra copies of data/descriptor fields |
| themselves in the process |
| vii.Ability to handle the possibility of splits/merges as the structure passes |
| through layered drivers (lvm, md, evms), with minimal overhead. |
| |
| The solution was to define a new structure (bio) for the block layer, |
| instead of using the buffer head structure (bh) directly, the idea being |
| avoidance of some associated baggage and limitations. The bio structure |
| is uniformly used for all i/o at the block layer ; it forms a part of the |
| bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are |
| mapped to bio structures. |
| |
| 2.2 The bio struct |
| |
| The bio structure uses a vector representation pointing to an array of tuples |
| of <page, offset, len> to describe the i/o buffer, and has various other |
| fields describing i/o parameters and state that needs to be maintained for |
| performing the i/o. |
| |
| Notice that this representation means that a bio has no virtual address |
| mapping at all (unlike buffer heads). |
| |
| struct bio_vec { |
| struct page *bv_page; |
| unsigned short bv_len; |
| unsigned short bv_offset; |
| }; |
| |
| /* |
| * main unit of I/O for the block layer and lower layers (ie drivers) |
| */ |
| struct bio { |
| sector_t bi_sector; |
| struct bio *bi_next; /* request queue link */ |
| struct block_device *bi_bdev; /* target device */ |
| unsigned long bi_flags; /* status, command, etc */ |
| unsigned long bi_rw; /* low bits: r/w, high: priority */ |
| |
| unsigned int bi_vcnt; /* how may bio_vec's */ |
| unsigned int bi_idx; /* current index into bio_vec array */ |
| |
| unsigned int bi_size; /* total size in bytes */ |
| unsigned short bi_phys_segments; /* segments after physaddr coalesce*/ |
| unsigned short bi_hw_segments; /* segments after DMA remapping */ |
| unsigned int bi_max; /* max bio_vecs we can hold |
| used as index into pool */ |
| struct bio_vec *bi_io_vec; /* the actual vec list */ |
| bio_end_io_t *bi_end_io; /* bi_end_io (bio) */ |
| atomic_t bi_cnt; /* pin count: free when it hits zero */ |
| void *bi_private; |
| }; |
| |
| With this multipage bio design: |
| |
| - Large i/os can be sent down in one go using a bio_vec list consisting |
| of an array of <page, offset, len> fragments (similar to the way fragments |
| are represented in the zero-copy network code) |
| - Splitting of an i/o request across multiple devices (as in the case of |
| lvm or raid) is achieved by cloning the bio (where the clone points to |
| the same bi_io_vec array, but with the index and size accordingly modified) |
| - A linked list of bios is used as before for unrelated merges (*) - this |
| avoids reallocs and makes independent completions easier to handle. |
| - Code that traverses the req list can find all the segments of a bio |
| by using rq_for_each_segment. This handles the fact that a request |
| has multiple bios, each of which can have multiple segments. |
| - Drivers which can't process a large bio in one shot can use the bi_idx |
| field to keep track of the next bio_vec entry to process. |
| (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE) |
| [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying |
| bi_offset an len fields] |
| |
| (*) unrelated merges -- a request ends up containing two or more bios that |
| didn't originate from the same place. |
| |
| bi_end_io() i/o callback gets called on i/o completion of the entire bio. |
| |
| At a lower level, drivers build a scatter gather list from the merged bios. |
| The scatter gather list is in the form of an array of <page, offset, len> |
| entries with their corresponding dma address mappings filled in at the |
| appropriate time. As an optimization, contiguous physical pages can be |
| covered by a single entry where <page> refers to the first page and <len> |
| covers the range of pages (up to 16 contiguous pages could be covered this |
| way). There is a helper routine (blk_rq_map_sg) which drivers can use to build |
| the sg list. |
| |
| Note: Right now the only user of bios with more than one page is ll_rw_kio, |
| which in turn means that only raw I/O uses it (direct i/o may not work |
| right now). The intent however is to enable clustering of pages etc to |
| become possible. The pagebuf abstraction layer from SGI also uses multi-page |
| bios, but that is currently not included in the stock development kernels. |
| The same is true of Andrew Morton's work-in-progress multipage bio writeout |
| and readahead patches. |
| |
| 2.3 Changes in the Request Structure |
| |
| The request structure is the structure that gets passed down to low level |
| drivers. The block layer make_request function builds up a request structure, |
| places it on the queue and invokes the drivers request_fn. The driver makes |
| use of block layer helper routine elv_next_request to pull the next request |
| off the queue. Control or diagnostic functions might bypass block and directly |
| invoke underlying driver entry points passing in a specially constructed |
| request structure. |
| |
| Only some relevant fields (mainly those which changed or may be referred |
| to in some of the discussion here) are listed below, not necessarily in |
| the order in which they occur in the structure (see include/linux/blkdev.h) |
| Refer to Documentation/block/request.txt for details about all the request |
| structure fields and a quick reference about the layers which are |
| supposed to use or modify those fields. |
| |
| struct request { |
| struct list_head queuelist; /* Not meant to be directly accessed by |
| the driver. |
| Used by q->elv_next_request_fn |
| rq->queue is gone |
| */ |
| . |
| . |
| unsigned char cmd[16]; /* prebuilt command data block */ |
| unsigned long flags; /* also includes earlier rq->cmd settings */ |
| . |
| . |
| sector_t sector; /* this field is now of type sector_t instead of int |
| preparation for 64 bit sectors */ |
| . |
| . |
| |
| /* Number of scatter-gather DMA addr+len pairs after |
| * physical address coalescing is performed. |
| */ |
| unsigned short nr_phys_segments; |
| |
| /* Number of scatter-gather addr+len pairs after |
| * physical and DMA remapping hardware coalescing is performed. |
| * This is the number of scatter-gather entries the driver |
| * will actually have to deal with after DMA mapping is done. |
| */ |
| unsigned short nr_hw_segments; |
| |
| /* Various sector counts */ |
| unsigned long nr_sectors; /* no. of sectors left: driver modifiable */ |
| unsigned long hard_nr_sectors; /* block internal copy of above */ |
| unsigned int current_nr_sectors; /* no. of sectors left in the |
| current segment:driver modifiable */ |
| unsigned long hard_cur_sectors; /* block internal copy of the above */ |
| . |
| . |
| int tag; /* command tag associated with request */ |
| void *special; /* same as before */ |
| char *buffer; /* valid only for low memory buffers up to |
| current_nr_sectors */ |
| . |
| . |
| struct bio *bio, *biotail; /* bio list instead of bh */ |
| struct request_list *rl; |
| } |
| |
| See the rq_flag_bits definitions for an explanation of the various flags |
| available. Some bits are used by the block layer or i/o scheduler. |
| |
| The behaviour of the various sector counts are almost the same as before, |
| except that since we have multi-segment bios, current_nr_sectors refers |
| to the numbers of sectors in the current segment being processed which could |
| be one of the many segments in the current bio (i.e i/o completion unit). |
| The nr_sectors value refers to the total number of sectors in the whole |
| request that remain to be transferred (no change). The purpose of the |
| hard_xxx values is for block to remember these counts every time it hands |
| over the request to the driver. These values are updated by block on |
| end_that_request_first, i.e. every time the driver completes a part of the |
| transfer and invokes block end*request helpers to mark this. The |
| driver should not modify these values. The block layer sets up the |
| nr_sectors and current_nr_sectors fields (based on the corresponding |
| hard_xxx values and the number of bytes transferred) and updates it on |
| every transfer that invokes end_that_request_first. It does the same for the |
| buffer, bio, bio->bi_idx fields too. |
| |
| The buffer field is just a virtual address mapping of the current segment |
| of the i/o buffer in cases where the buffer resides in low-memory. For high |
| memory i/o, this field is not valid and must not be used by drivers. |
| |
| Code that sets up its own request structures and passes them down to |
| a driver needs to be careful about interoperation with the block layer helper |
| functions which the driver uses. (Section 1.3) |
| |
| 3. Using bios |
| |
| 3.1 Setup/Teardown |
| |
| There are routines for managing the allocation, and reference counting, and |
| freeing of bios (bio_alloc, bio_get, bio_put). |
| |
| This makes use of Ingo Molnar's mempool implementation, which enables |
| subsystems like bio to maintain their own reserve memory pools for guaranteed |
| deadlock-free allocations during extreme VM load. For example, the VM |
| subsystem makes use of the block layer to writeout dirty pages in order to be |
| able to free up memory space, a case which needs careful handling. The |
| allocation logic draws from the preallocated emergency reserve in situations |
| where it cannot allocate through normal means. If the pool is empty and it |
| can wait, then it would trigger action that would help free up memory or |
| replenish the pool (without deadlocking) and wait for availability in the pool. |
| If it is in IRQ context, and hence not in a position to do this, allocation |
| could fail if the pool is empty. In general mempool always first tries to |
| perform allocation without having to wait, even if it means digging into the |
| pool as long it is not less that 50% full. |
| |
| On a free, memory is released to the pool or directly freed depending on |
| the current availability in the pool. The mempool interface lets the |
| subsystem specify the routines to be used for normal alloc and free. In the |
| case of bio, these routines make use of the standard slab allocator. |
| |
| The caller of bio_alloc is expected to taken certain steps to avoid |
| deadlocks, e.g. avoid trying to allocate more memory from the pool while |
| already holding memory obtained from the pool. |
| [TBD: This is a potential issue, though a rare possibility |
| in the bounce bio allocation that happens in the current code, since |
| it ends up allocating a second bio from the same pool while |
| holding the original bio ] |
| |
| Memory allocated from the pool should be released back within a limited |
| amount of time (in the case of bio, that would be after the i/o is completed). |
| This ensures that if part of the pool has been used up, some work (in this |
| case i/o) must already be in progress and memory would be available when it |
| is over. If allocating from multiple pools in the same code path, the order |
| or hierarchy of allocation needs to be consistent, just the way one deals |
| with multiple locks. |
| |
| The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc()) |
| for a non-clone bio. There are the 6 pools setup for different size biovecs, |
| so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the |
| given size from these slabs. |
| |
| The bio_get() routine may be used to hold an extra reference on a bio prior |
| to i/o submission, if the bio fields are likely to be accessed after the |
| i/o is issued (since the bio may otherwise get freed in case i/o completion |
| happens in the meantime). |
| |
| The bio_clone() routine may be used to duplicate a bio, where the clone |
| shares the bio_vec_list with the original bio (i.e. both point to the |
| same bio_vec_list). This would typically be used for splitting i/o requests |
| in lvm or md. |
| |
| 3.2 Generic bio helper Routines |
| |
| 3.2.1 Traversing segments and completion units in a request |
| |
| The macro rq_for_each_segment() should be used for traversing the bios |
| in the request list (drivers should avoid directly trying to do it |
| themselves). Using these helpers should also make it easier to cope |
| with block changes in the future. |
| |
| struct req_iterator iter; |
| rq_for_each_segment(bio_vec, rq, iter) |
| /* bio_vec is now current segment */ |
| |
| I/O completion callbacks are per-bio rather than per-segment, so drivers |
| that traverse bio chains on completion need to keep that in mind. Drivers |
| which don't make a distinction between segments and completion units would |
| need to be reorganized to support multi-segment bios. |
| |
| 3.2.2 Setting up DMA scatterlists |
| |
| The blk_rq_map_sg() helper routine would be used for setting up scatter |
| gather lists from a request, so a driver need not do it on its own. |
| |
| nr_segments = blk_rq_map_sg(q, rq, scatterlist); |
| |
| The helper routine provides a level of abstraction which makes it easier |
| to modify the internals of request to scatterlist conversion down the line |
| without breaking drivers. The blk_rq_map_sg routine takes care of several |
| things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER |
| is set) and correct segment accounting to avoid exceeding the limits which |
| the i/o hardware can handle, based on various queue properties. |
| |
| - Prevents a clustered segment from crossing a 4GB mem boundary |
| - Avoids building segments that would exceed the number of physical |
| memory segments that the driver can handle (phys_segments) and the |
| number that the underlying hardware can handle at once, accounting for |
| DMA remapping (hw_segments) (i.e. IOMMU aware limits). |
| |
| Routines which the low level driver can use to set up the segment limits: |
| |
| blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of |
| hw data segments in a request (i.e. the maximum number of address/length |
| pairs the host adapter can actually hand to the device at once) |
| |
| blk_queue_max_phys_segments() : Sets an upper limit on the maximum number |
| of physical data segments in a request (i.e. the largest sized scatter list |
| a driver could handle) |
| |
| 3.2.3 I/O completion |
| |
| The existing generic block layer helper routines end_request, |
| end_that_request_first and end_that_request_last can be used for i/o |
| completion (and setting things up so the rest of the i/o or the next |
| request can be kicked of) as before. With the introduction of multi-page |
| bio support, end_that_request_first requires an additional argument indicating |
| the number of sectors completed. |
| |
| 3.2.4 Implications for drivers that do not interpret bios (don't handle |
| multiple segments) |
| |
| Drivers that do not interpret bios e.g those which do not handle multiple |
| segments and do not support i/o into high memory addresses (require bounce |
| buffers) and expect only virtually mapped buffers, can access the rq->buffer |
| field. As before the driver should use current_nr_sectors to determine the |
| size of remaining data in the current segment (that is the maximum it can |
| transfer in one go unless it interprets segments), and rely on the block layer |
| end_request, or end_that_request_first/last to take care of all accounting |
| and transparent mapping of the next bio segment when a segment boundary |
| is crossed on completion of a transfer. (The end*request* functions should |
| be used if only if the request has come down from block/bio path, not for |
| direct access requests which only specify rq->buffer without a valid rq->bio) |
| |
| 3.2.5 Generic request command tagging |
| |
| 3.2.5.1 Tag helpers |
| |
| Block now offers some simple generic functionality to help support command |
| queueing (typically known as tagged command queueing), ie manage more than |
| one outstanding command on a queue at any given time. |
| |
| blk_queue_init_tags(struct request_queue *q, int depth) |
| |
| Initialize internal command tagging structures for a maximum |
| depth of 'depth'. |
| |
| blk_queue_free_tags((struct request_queue *q) |
| |
| Teardown tag info associated with the queue. This will be done |
| automatically by block if blk_queue_cleanup() is called on a queue |
| that is using tagging. |
| |
| The above are initialization and exit management, the main helpers during |
| normal operations are: |
| |
| blk_queue_start_tag(struct request_queue *q, struct request *rq) |
| |
| Start tagged operation for this request. A free tag number between |
| 0 and 'depth' is assigned to the request (rq->tag holds this number), |
| and 'rq' is added to the internal tag management. If the maximum depth |
| for this queue is already achieved (or if the tag wasn't started for |
| some other reason), 1 is returned. Otherwise 0 is returned. |
| |
| blk_queue_end_tag(struct request_queue *q, struct request *rq) |
| |
| End tagged operation on this request. 'rq' is removed from the internal |
| book keeping structures. |
| |
| To minimize struct request and queue overhead, the tag helpers utilize some |
| of the same request members that are used for normal request queue management. |
| This means that a request cannot both be an active tag and be on the queue |
| list at the same time. blk_queue_start_tag() will remove the request, but |
| the driver must remember to call blk_queue_end_tag() before signalling |
| completion of the request to the block layer. This means ending tag |
| operations before calling end_that_request_last()! For an example of a user |
| of these helpers, see the IDE tagged command queueing support. |
| |
| Certain hardware conditions may dictate a need to invalidate the block tag |
| queue. For instance, on IDE any tagged request error needs to clear both |
| the hardware and software block queue and enable the driver to sanely restart |
| all the outstanding requests. There's a third helper to do that: |
| |
| blk_queue_invalidate_tags(struct request_queue *q) |
| |
| Clear the internal block tag queue and re-add all the pending requests |
| to the request queue. The driver will receive them again on the |
| next request_fn run, just like it did the first time it encountered |
| them. |
| |
| 3.2.5.2 Tag info |
| |
| Some block functions exist to query current tag status or to go from a |
| tag number to the associated request. These are, in no particular order: |
| |
| blk_queue_tagged(q) |
| |
| Returns 1 if the queue 'q' is using tagging, 0 if not. |
| |
| blk_queue_tag_request(q, tag) |
| |
| Returns a pointer to the request associated with tag 'tag'. |
| |
| blk_queue_tag_depth(q) |
| |
| Return current queue depth. |
| |
| blk_queue_tag_queue(q) |
| |
| Returns 1 if the queue can accept a new queued command, 0 if we are |
| at the maximum depth already. |
| |
| blk_queue_rq_tagged(rq) |
| |
| Returns 1 if the request 'rq' is tagged. |
| |
| 3.2.5.2 Internal structure |
| |
| Internally, block manages tags in the blk_queue_tag structure: |
| |
| struct blk_queue_tag { |
| struct request **tag_index; /* array or pointers to rq */ |
| unsigned long *tag_map; /* bitmap of free tags */ |
| struct list_head busy_list; /* fifo list of busy tags */ |
| int busy; /* queue depth */ |
| int max_depth; /* max queue depth */ |
| }; |
| |
| Most of the above is simple and straight forward, however busy_list may need |
| a bit of explaining. Normally we don't care too much about request ordering, |
| but in the event of any barrier requests in the tag queue we need to ensure |
| that requests are restarted in the order they were queue. This may happen |
| if the driver needs to use blk_queue_invalidate_tags(). |
| |
| Tagging also defines a new request flag, REQ_QUEUED. This is set whenever |
| a request is currently tagged. You should not use this flag directly, |
| blk_rq_tagged(rq) is the portable way to do so. |
| |
| 3.3 I/O Submission |
| |
| The routine submit_bio() is used to submit a single io. Higher level i/o |
| routines make use of this: |
| |
| (a) Buffered i/o: |
| The routine submit_bh() invokes submit_bio() on a bio corresponding to the |
| bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before. |
| |
| (b) Kiobuf i/o (for raw/direct i/o): |
| The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and |
| maps the array to one or more multi-page bios, issuing submit_bio() to |
| perform the i/o on each of these. |
| |
| The embedded bh array in the kiobuf structure has been removed and no |
| preallocation of bios is done for kiobufs. [The intent is to remove the |
| blocks array as well, but it's currently in there to kludge around direct i/o.] |
| Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc. |
| |
| Todo/Observation: |
| |
| A single kiobuf structure is assumed to correspond to a contiguous range |
| of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec. |
| So right now it wouldn't work for direct i/o on non-contiguous blocks. |
| This is to be resolved. The eventual direction is to replace kiobuf |
| by kvec's. |
| |
| Badari Pulavarty has a patch to implement direct i/o correctly using |
| bio and kvec. |
| |
| |
| (c) Page i/o: |
| Todo/Under discussion: |
| |
| Andrew Morton's multi-page bio patches attempt to issue multi-page |
| writeouts (and reads) from the page cache, by directly building up |
| large bios for submission completely bypassing the usage of buffer |
| heads. This work is still in progress. |
| |
| Christoph Hellwig had some code that uses bios for page-io (rather than |
| bh). This isn't included in bio as yet. Christoph was also working on a |
| design for representing virtual/real extents as an entity and modifying |
| some of the address space ops interfaces to utilize this abstraction rather |
| than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf |
| abstraction, but intended to be as lightweight as possible). |
| |
| (d) Direct access i/o: |
| Direct access requests that do not contain bios would be submitted differently |
| as discussed earlier in section 1.3. |
| |
| Aside: |
| |
| Kvec i/o: |
| |
| Ben LaHaise's aio code uses a slightly different structure instead |
| of kiobufs, called a kvec_cb. This contains an array of <page, offset, len> |
| tuples (very much like the networking code), together with a callback function |
| and data pointer. This is embedded into a brw_cb structure when passed |
| to brw_kvec_async(). |
| |
| Now it should be possible to directly map these kvecs to a bio. Just as while |
| cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec |
| array pointer to point to the veclet array in kvecs. |
| |
| TBD: In order for this to work, some changes are needed in the way multi-page |
| bios are handled today. The values of the tuples in such a vector passed in |
| from higher level code should not be modified by the block layer in the course |
| of its request processing, since that would make it hard for the higher layer |
| to continue to use the vector descriptor (kvec) after i/o completes. Instead, |
| all such transient state should either be maintained in the request structure, |
| and passed on in some way to the endio completion routine. |
| |
| |
| 4. The I/O scheduler |
| I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch |
| queue and specific I/O schedulers. Unless stated otherwise, elevator is used |
| to refer to both parts and I/O scheduler to specific I/O schedulers. |
| |
| Block layer implements generic dispatch queue in block/*.c. |
| The generic dispatch queue is responsible for properly ordering barrier |
| requests, requeueing, handling non-fs requests and all other subtleties. |
| |
| Specific I/O schedulers are responsible for ordering normal filesystem |
| requests. They can also choose to delay certain requests to improve |
| throughput or whatever purpose. As the plural form indicates, there are |
| multiple I/O schedulers. They can be built as modules but at least one should |
| be built inside the kernel. Each queue can choose different one and can also |
| change to another one dynamically. |
| |
| A block layer call to the i/o scheduler follows the convention elv_xxx(). This |
| calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx |
| and xxx might not match exactly, but use your imagination. If an elevator |
| doesn't implement a function, the switch does nothing or some minimal house |
| keeping work. |
| |
| 4.1. I/O scheduler API |
| |
| The functions an elevator may implement are: (* are mandatory) |
| elevator_merge_fn called to query requests for merge with a bio |
| |
| elevator_merge_req_fn called when two requests get merged. the one |
| which gets merged into the other one will be |
| never seen by I/O scheduler again. IOW, after |
| being merged, the request is gone. |
| |
| elevator_merged_fn called when a request in the scheduler has been |
| involved in a merge. It is used in the deadline |
| scheduler for example, to reposition the request |
| if its sorting order has changed. |
| |
| elevator_allow_merge_fn called whenever the block layer determines |
| that a bio can be merged into an existing |
| request safely. The io scheduler may still |
| want to stop a merge at this point if it |
| results in some sort of conflict internally, |
| this hook allows it to do that. |
| |
| elevator_dispatch_fn* fills the dispatch queue with ready requests. |
| I/O schedulers are free to postpone requests by |
| not filling the dispatch queue unless @force |
| is non-zero. Once dispatched, I/O schedulers |
| are not allowed to manipulate the requests - |
| they belong to generic dispatch queue. |
| |
| elevator_add_req_fn* called to add a new request into the scheduler |
| |
| elevator_former_req_fn |
| elevator_latter_req_fn These return the request before or after the |
| one specified in disk sort order. Used by the |
| block layer to find merge possibilities. |
| |
| elevator_completed_req_fn called when a request is completed. |
| |
| elevator_may_queue_fn returns true if the scheduler wants to allow the |
| current context to queue a new request even if |
| it is over the queue limit. This must be used |
| very carefully!! |
| |
| elevator_set_req_fn |
| elevator_put_req_fn Must be used to allocate and free any elevator |
| specific storage for a request. |
| |
| elevator_activate_req_fn Called when device driver first sees a request. |
| I/O schedulers can use this callback to |
| determine when actual execution of a request |
| starts. |
| elevator_deactivate_req_fn Called when device driver decides to delay |
| a request by requeueing it. |
| |
| elevator_init_fn* |
| elevator_exit_fn Allocate and free any elevator specific storage |
| for a queue. |
| |
| 4.2 Request flows seen by I/O schedulers |
| All requests seen by I/O schedulers strictly follow one of the following three |
| flows. |
| |
| set_req_fn -> |
| |
| i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn -> |
| (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn |
| ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn |
| iii. [none] |
| |
| -> put_req_fn |
| |
| 4.3 I/O scheduler implementation |
| The generic i/o scheduler algorithm attempts to sort/merge/batch requests for |
| optimal disk scan and request servicing performance (based on generic |
| principles and device capabilities), optimized for: |
| i. improved throughput |
| ii. improved latency |
| iii. better utilization of h/w & CPU time |
| |
| Characteristics: |
| |
| i. Binary tree |
| AS and deadline i/o schedulers use red black binary trees for disk position |
| sorting and searching, and a fifo linked list for time-based searching. This |
| gives good scalability and good availability of information. Requests are |
| almost always dispatched in disk sort order, so a cache is kept of the next |
| request in sort order to prevent binary tree lookups. |
| |
| This arrangement is not a generic block layer characteristic however, so |
| elevators may implement queues as they please. |
| |
| ii. Merge hash |
| AS and deadline use a hash table indexed by the last sector of a request. This |
| enables merging code to quickly look up "back merge" candidates, even when |
| multiple I/O streams are being performed at once on one disk. |
| |
| "Front merges", a new request being merged at the front of an existing request, |
| are far less common than "back merges" due to the nature of most I/O patterns. |
| Front merges are handled by the binary trees in AS and deadline schedulers. |
| |
| iii. Plugging the queue to batch requests in anticipation of opportunities for |
| merge/sort optimizations |
| |
| Plugging is an approach that the current i/o scheduling algorithm resorts to so |
| that it collects up enough requests in the queue to be able to take |
| advantage of the sorting/merging logic in the elevator. If the |
| queue is empty when a request comes in, then it plugs the request queue |
| (sort of like plugging the bath tub of a vessel to get fluid to build up) |
| till it fills up with a few more requests, before starting to service |
| the requests. This provides an opportunity to merge/sort the requests before |
| passing them down to the device. There are various conditions when the queue is |
| unplugged (to open up the flow again), either through a scheduled task or |
| could be on demand. For example wait_on_buffer sets the unplugging going |
| through sync_buffer() running blk_run_address_space(mapping). Or the caller |
| can do it explicity through blk_unplug(bdev). So in the read case, |
| the queue gets explicitly unplugged as part of waiting for completion on that |
| buffer. For page driven IO, the address space ->sync_page() takes care of |
| doing the blk_run_address_space(). |
| |
| Aside: |
| This is kind of controversial territory, as it's not clear if plugging is |
| always the right thing to do. Devices typically have their own queues, |
| and allowing a big queue to build up in software, while letting the device be |
| idle for a while may not always make sense. The trick is to handle the fine |
| balance between when to plug and when to open up. Also now that we have |
| multi-page bios being queued in one shot, we may not need to wait to merge |
| a big request from the broken up pieces coming by. |
| |
| 4.4 I/O contexts |
| I/O contexts provide a dynamically allocated per process data area. They may |
| be used in I/O schedulers, and in the block layer (could be used for IO statis, |
| priorities for example). See *io_context in block/ll_rw_blk.c, and as-iosched.c |
| for an example of usage in an i/o scheduler. |
| |
| |
| 5. Scalability related changes |
| |
| 5.1 Granular Locking: io_request_lock replaced by a per-queue lock |
| |
| The global io_request_lock has been removed as of 2.5, to avoid |
| the scalability bottleneck it was causing, and has been replaced by more |
| granular locking. The request queue structure has a pointer to the |
| lock to be used for that queue. As a result, locking can now be |
| per-queue, with a provision for sharing a lock across queues if |
| necessary (e.g the scsi layer sets the queue lock pointers to the |
| corresponding adapter lock, which results in a per host locking |
| granularity). The locking semantics are the same, i.e. locking is |
| still imposed by the block layer, grabbing the lock before |
| request_fn execution which it means that lots of older drivers |
| should still be SMP safe. Drivers are free to drop the queue |
| lock themselves, if required. Drivers that explicitly used the |
| io_request_lock for serialization need to be modified accordingly. |
| Usually it's as easy as adding a global lock: |
| |
| static DEFINE_SPINLOCK(my_driver_lock); |
| |
| and passing the address to that lock to blk_init_queue(). |
| |
| 5.2 64 bit sector numbers (sector_t prepares for 64 bit support) |
| |
| The sector number used in the bio structure has been changed to sector_t, |
| which could be defined as 64 bit in preparation for 64 bit sector support. |
| |
| 6. Other Changes/Implications |
| |
| 6.1 Partition re-mapping handled by the generic block layer |
| |
| In 2.5 some of the gendisk/partition related code has been reorganized. |
| Now the generic block layer performs partition-remapping early and thus |
| provides drivers with a sector number relative to whole device, rather than |
| having to take partition number into account in order to arrive at the true |
| sector number. The routine blk_partition_remap() is invoked by |
| generic_make_request even before invoking the queue specific make_request_fn, |
| so the i/o scheduler also gets to operate on whole disk sector numbers. This |
| should typically not require changes to block drivers, it just never gets |
| to invoke its own partition sector offset calculations since all bios |
| sent are offset from the beginning of the device. |
| |
| |
| 7. A Few Tips on Migration of older drivers |
| |
| Old-style drivers that just use CURRENT and ignores clustered requests, |
| may not need much change. The generic layer will automatically handle |
| clustered requests, multi-page bios, etc for the driver. |
| |
| For a low performance driver or hardware that is PIO driven or just doesn't |
| support scatter-gather changes should be minimal too. |
| |
| The following are some points to keep in mind when converting old drivers |
| to bio. |
| |
| Drivers should use elv_next_request to pick up requests and are no longer |
| supposed to handle looping directly over the request list. |
| (struct request->queue has been removed) |
| |
| Now end_that_request_first takes an additional number_of_sectors argument. |
| It used to handle always just the first buffer_head in a request, now |
| it will loop and handle as many sectors (on a bio-segment granularity) |
| as specified. |
| |
| Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the |
| right thing to use is bio_endio(bio, uptodate) instead. |
| |
| If the driver is dropping the io_request_lock from its request_fn strategy, |
| then it just needs to replace that with q->queue_lock instead. |
| |
| As described in Sec 1.1, drivers can set max sector size, max segment size |
| etc per queue now. Drivers that used to define their own merge functions i |
| to handle things like this can now just use the blk_queue_* functions at |
| blk_init_queue time. |
| |
| Drivers no longer have to map a {partition, sector offset} into the |
| correct absolute location anymore, this is done by the block layer, so |
| where a driver received a request ala this before: |
| |
| rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */ |
| rq->sector = 0; /* first sector on hda5 */ |
| |
| it will now see |
| |
| rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */ |
| rq->sector = 123128; /* offset from start of disk */ |
| |
| As mentioned, there is no virtual mapping of a bio. For DMA, this is |
| not a problem as the driver probably never will need a virtual mapping. |
| Instead it needs a bus mapping (dma_map_page for a single segment or |
| use dma_map_sg for scatter gather) to be able to ship it to the driver. For |
| PIO drivers (or drivers that need to revert to PIO transfer once in a |
| while (IDE for example)), where the CPU is doing the actual data |
| transfer a virtual mapping is needed. If the driver supports highmem I/O, |
| (Sec 1.1, (ii) ) it needs to use __bio_kmap_atomic and bio_kmap_irq to |
| temporarily map a bio into the virtual address space. |
| |
| |
| 8. Prior/Related/Impacted patches |
| |
| 8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp) |
| - orig kiobuf & raw i/o patches (now in 2.4 tree) |
| - direct kiobuf based i/o to devices (no intermediate bh's) |
| - page i/o using kiobuf |
| - kiobuf splitting for lvm (mkp) |
| - elevator support for kiobuf request merging (axboe) |
| 8.2. Zero-copy networking (Dave Miller) |
| 8.3. SGI XFS - pagebuf patches - use of kiobufs |
| 8.4. Multi-page pioent patch for bio (Christoph Hellwig) |
| 8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11 |
| 8.6. Async i/o implementation patch (Ben LaHaise) |
| 8.7. EVMS layering design (IBM EVMS team) |
| 8.8. Larger page cache size patch (Ben LaHaise) and |
| Large page size (Daniel Phillips) |
| => larger contiguous physical memory buffers |
| 8.9. VM reservations patch (Ben LaHaise) |
| 8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?) |
| 8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+ |
| 8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, |
| Badari) |
| 8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven) |
| 8.14 IDE Taskfile i/o patch (Andre Hedrick) |
| 8.15 Multi-page writeout and readahead patches (Andrew Morton) |
| 8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy) |
| |
| 9. Other References: |
| |
| 9.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml, |
| and Linus' comments - Jan 2001) |
| 9.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan |
| et al - Feb-March 2001 (many of the initial thoughts that led to bio were |
| brought up in this discussion thread) |
| 9.3 Discussions on mempool on lkml - Dec 2001. |
| |