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block, drivers, fs: rename REQ_FLUSH to REQ_PREFLUSH

Browse source 
To avoid confusion between REQ_OP_FLUSH, which is handled by
request_fn drivers, and upper layers requesting the block layer
perform a flush sequence along with possibly a WRITE, this patch
renames REQ_FLUSH to REQ_PREFLUSH.

Signed-off-by: Mike Christie <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Mike Christie 2016-06-05 14:32:25 -05:00 • committed by Jens Axboe
commit 28a8f0d317
39 changed files with 102 additions and 98 deletions
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22
Documentation/block/writeback_cache_control.txt
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@ -20,11 +20,11 @@ a forced cache flush, and the Force Unit Access (FUA) flag for requests.
Explicit cache flushes
----------------------
The REQ_FLUSH flag can be OR ed into the r/w flags of a bio submitted from
The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from
the filesystem and will make sure the volatile cache of the storage device
has been flushed before the actual I/O operation is started. This explicitly
guarantees that previously completed write requests are on non-volatile
storage before the flagged bio starts. In addition the REQ_FLUSH flag can be
storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be
set on an otherwise empty bio structure, which causes only an explicit cache
flush without any dependent I/O. It is recommend to use
the blkdev_issue_flush() helper for a pure cache flush.
@ -41,21 +41,21 @@ signaled after the data has been committed to non-volatile storage.
Implementation details for filesystems
--------------------------------------
Filesystems can simply set the REQ_FLUSH and REQ_FUA bits and do not have to
Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to
worry if the underlying devices need any explicit cache flushing and how
the Forced Unit Access is implemented. The REQ_FLUSH and REQ_FUA flags
the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags
may both be set on a single bio.
Implementation details for make_request_fn based block drivers
--------------------------------------------------------------
These drivers will always see the REQ_FLUSH and REQ_FUA bits as they sit
These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit
directly below the submit_bio interface. For remapping drivers the REQ_FUA
bits need to be propagated to underlying devices, and a global flush needs
to be implemented for bios with the REQ_FLUSH bit set. For real device
drivers that do not have a volatile cache the REQ_FLUSH and REQ_FUA bits
on non-empty bios can simply be ignored, and REQ_FLUSH requests without
to be implemented for bios with the REQ_PREFLUSH bit set. For real device
drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits
on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without
data can be completed successfully without doing any work. Drivers for
devices with volatile caches need to implement the support for these
flags themselves without any help from the block layer.
@ -65,8 +65,8 @@ Implementation details for request_fn based block drivers
--------------------------------------------------------------
For devices that do not support volatile write caches there is no driver
support required, the block layer completes empty REQ_FLUSH requests before
entering the driver and strips off the REQ_FLUSH and REQ_FUA bits from
support required, the block layer completes empty REQ_PREFLUSH requests before
entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from
requests that have a payload. For devices with volatile write caches the
driver needs to tell the block layer that it supports flushing caches by
doing:
@ -74,7 +74,7 @@ doing:
blk_queue_write_cache(sdkp->disk->queue, true, false);
and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that
REQ_FLUSH requests with a payload are automatically turned into a sequence
REQ_PREFLUSH requests with a payload are automatically turned into a sequence
of an empty REQ_OP_FLUSH request followed by the actual write by the block
layer. For devices that also support the FUA bit the block layer needs
to be told to pass through the REQ_FUA bit using: