linux-kernel - Re: merging discard request in the block layer

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Date:	Tue, 22 Mar 2011 17:00:34 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: merging discard request in the block layer

On Tue, Mar 22, 2011 at 08:54:06PM +0100, Jens Axboe wrote:
> > 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;
> >  
> >  	/*
> 
> That's not going to be enough, you want to disable the bio to request
> merging of discards as well in elevator.c:elv_rq_merge_ok(). Does
> that then fix it?

Applying the same fix in elv_rq_merge_ok seems to fix the issue, at
least the xfstests testcase that usually hits it is completes ok.

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