linux-kernel - merging discard request in the block layer

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Message-ID: <20110322194755.GA20122@infradead.org>
Date:	Tue, 22 Mar 2011 15:47:55 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	linux-kernel@...r.kernel.org
Subject: merging discard request in the block layer

It seems the current block layer wil happily try to merge discard
requests that were split because they are at the max that bi_size
can hold together again.  At least that's what the

	blk: request botched

make me believe when testing XFS code that allows multiple
asynchronous discard request, unlike the current blkdev_issue_discard
which always waits for one before starting the next.

I tried this little sniplet to prevent it:

Index: xfs/block/blk-merge.c
===================================================================
--- xfs.orig/block/blk-merge.c	2011-03-22 13:07:24.733857580 +0100
+++ xfs/block/blk-merge.c	2011-03-22 13:08:17.448856577 +0100
@@ -373,7 +373,7 @@ static int attempt_merge(struct request_
 	/*
 	 * Don't merge file system requests and discard requests
 	 */
-	if ((req->cmd_flags & REQ_DISCARD) != (next->cmd_flags & REQ_DISCARD))
+	if ((req->cmd_flags & REQ_DISCARD) || (next->cmd_flags & REQ_DISCARD))
 		return 0;
 
 	/*

but it has no effect.  Using the big hammer and bypassing the whole
I/O schedule logic on the other works fine:

Index: xfs/block/blk-core.c
===================================================================
--- xfs.orig/block/blk-core.c	2011-03-22 13:07:24.717855861 +0100
+++ xfs/block/blk-core.c	2011-03-22 14:56:13.424856289 +0100
@@ -1218,7 +1218,7 @@ static int __make_request(struct request
 
 	spin_lock_irq(q->queue_lock);
 
-	if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
+	if (bio->bi_rw & (REQ_FLUSH | REQ_FUA | REQ_DISCARD)) {
 		where = ELEVATOR_INSERT_FRONT;
 		goto get_rq;
 	}
--
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