Commit ab0fd1de authored Jul 03, 2009 by Tejun Heo Committed by Jens Axboe Jul 03, 2009

block: don't merge requests of different failfast settings

Block layer used to merge requests and bios with different failfast
settings.  This caused regular IOs to fail prematurely when they were
merged into failfast requests for readahead.

Niel Lambrechts could trigger the problem semi-reliably on ext4 when
resuming from STR.  ext4 uses readahead when reading inodes and
combined with the deterministic extra SATA PHY exception cycle during
resume on the specific configuration, non-readahead inode read would
fail causing ext4 errors.  Please read the following thread for
details.

  http://lkml.org/lkml/2009/5/23/21



This patch makes block layer reject merging if the failfast settings
don't match.  This is correct but likely to lower IO performance by
preventing regular IOs from mingling into surrounding readahead
requests.  Changes to allow such mixed merges and handle errors
correctly will be added later.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Niel Lambrechts <niel.lambrechts@gmail.com>
Cc: Theodore Tso <tytso@mit.edu>
Signed-off-by: Jens Axboe <axboe@carl.(none)>

parent b59e64d0

block/blk-merge.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -350,6 +350,12 @@ static int attempt_merge(struct request_queue q, struct request req,
		if (blk_integrity_rq(req) != blk_integrity_rq(next))
		return 0;

		/* don't merge requests of different failfast settings */
		if (blk_failfast_dev(req) != blk_failfast_dev(next) \|\|
		blk_failfast_transport(req) != blk_failfast_transport(next) \|\|
		blk_failfast_driver(req) != blk_failfast_driver(next))
		return 0;

		/*
		* If we are allowed to merge, then append bio list
		* from next to rq and release next. merge_requests_fn

block/elevator.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -100,6 +100,14 @@ int elv_rq_merge_ok(struct request rq, struct bio bio)
		if (bio_integrity(bio) != blk_integrity_rq(rq))
		return 0;

		/*
		* Don't merge if failfast settings don't match
		*/
		if (bio_failfast_dev(bio) != blk_failfast_dev(rq) \|\|
		bio_failfast_transport(bio) != blk_failfast_transport(rq) \|\|
		bio_failfast_driver(bio) != blk_failfast_driver(rq))
		return 0;

		if (!elv_iosched_allow_merge(rq, bio))
		return 0;

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