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)>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 39ce64432ba6d4cdcff6427851fe19aac172a2d7..e1999679a4d5156b967729c5e2b6b229a8747954 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -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

diff --git a/block/elevator.c b/block/elevator.c
index ca861927ba41b2dfc9ff80c139f6c8cc162fc251..6f2375339a996a794c17c96d0f32aa724fd1fae2 100644 (file)
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -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;