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block: prevent merges of discard and write requests

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Add logic to prevent two I/O requests being merged if
only one of them is a discard.  Ditto secure discard.

Without this fix, it is possible for write requests
to transform into discard requests.  For example:

  Submit bio 1 to discard 8 sectors from sector n
  Submit bio 2 to write 8 sectors from sector n + 16
  Submit bio 3 to write 8 sectors from sector n + 8

Bio 1 becomes request 1.  Bio 2 becomes request 2.
Bio 3 is merged with request 2, and then subsequently
request 2 is merged with request 1 resulting in just
one I/O request which discards all 24 sectors.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>

(Moved the checks above the position checks /Jens)

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
This commit is contained in:
Adrian Hunter 2010-09-25 12:42:55 +02:00 committed by Jens Axboe
parent a850ea3037
commit f281fb5fe5
1 changed files with 12 additions and 0 deletions
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12
block/blk-merge.c
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@ -361,6 +361,18 @@ static int attempt_merge(struct request_queue *q, struct request *req,
if (!rq_mergeable(req) || !rq_mergeable(next))
return 0;
/*
* Don't merge file system requests and discard requests
*/
if ((req->cmd_flags & REQ_DISCARD) != (next->cmd_flags & REQ_DISCARD))
return 0;
/*
* Don't merge discard requests and secure discard requests
*/
if ((req->cmd_flags & REQ_SECURE) != (next->cmd_flags & REQ_SECURE))
return 0;
/*
* not contiguous
*/