Document and move the various READ/WRITE types
It's a somewhat twisty maze of hints and behavioural modifiers, try
and clear it up a bit with some documentation.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 562d285..b535aec4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -87,6 +87,60 @@
*/
#define FMODE_NOCMTIME ((__force fmode_t)2048)
+/*
+ * The below are the various read and write types that we support. Some of
+ * them include behavioral modifiers that send information down to the
+ * block layer and IO scheduler. Terminology:
+ *
+ * The block layer uses device plugging to defer IO a little bit, in
+ * the hope that we will see more IO very shortly. This increases
+ * coalescing of adjacent IO and thus reduces the number of IOs we
+ * have to send to the device. It also allows for better queuing,
+ * if the IO isn't mergeable. If the caller is going to be waiting
+ * for the IO, then he must ensure that the device is unplugged so
+ * that the IO is dispatched to the driver.
+ *
+ * All IO is handled async in Linux. This is fine for background
+ * writes, but for reads or writes that someone waits for completion
+ * on, we want to notify the block layer and IO scheduler so that they
+ * know about it. That allows them to make better scheduling
+ * decisions. So when the below references 'sync' and 'async', it
+ * is referencing this priority hint.
+ *
+ * With that in mind, the available types are:
+ *
+ * READ A normal read operation. Device will be plugged.
+ * READ_SYNC A synchronous read. Device is not plugged, caller can
+ * immediately wait on this read without caring about
+ * unplugging.
+ * READA Used for read-ahead operations. Lower priority, and the
+ * block layer could (in theory) choose to ignore this
+ * request if it runs into resource problems.
+ * WRITE A normal async write. Device will be plugged.
+ * SWRITE Like WRITE, but a special case for ll_rw_block() that
+ * tells it to lock the buffer first. Normally a buffer
+ * must be locked before doing IO.
+ * WRITE_SYNC_PLUG Synchronous write. Identical to WRITE, but passes down
+ * the hint that someone will be waiting on this IO
+ * shortly. The device must still be unplugged explicitly,
+ * WRITE_SYNC_PLUG does not do this as we could be
+ * submitting more writes before we actually wait on any
+ * of them.
+ * WRITE_SYNC Like WRITE_SYNC_PLUG, but also unplugs the device
+ * immediately after submission. The write equivalent
+ * of READ_SYNC.
+ * WRITE_ODIRECT Special case write for O_DIRECT only.
+ * SWRITE_SYNC
+ * SWRITE_SYNC_PLUG Like WRITE_SYNC/WRITE_SYNC_PLUG, but locks the buffer.
+ * See SWRITE.
+ * WRITE_BARRIER Like WRITE, but tells the block layer that all
+ * previously submitted writes must be safely on storage
+ * before this one is started. Also guarantees that when
+ * this write is complete, it itself is also safely on
+ * storage. Prevents reordering of writes on both sides
+ * of this IO.
+ *
+ */
#define RW_MASK 1
#define RWA_MASK 2
#define READ 0
@@ -102,6 +156,11 @@
(SWRITE | (1 << BIO_RW_SYNCIO) | (1 << BIO_RW_NOIDLE))
#define SWRITE_SYNC (SWRITE_SYNC_PLUG | (1 << BIO_RW_UNPLUG))
#define WRITE_BARRIER (WRITE | (1 << BIO_RW_BARRIER))
+
+/*
+ * These aren't really reads or writes, they pass down information about
+ * parts of device that are now unused by the file system.
+ */
#define DISCARD_NOBARRIER (1 << BIO_RW_DISCARD)
#define DISCARD_BARRIER ((1 << BIO_RW_DISCARD) | (1 << BIO_RW_BARRIER))