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

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Date:	Tue, 22 Mar 2011 20:54:06 +0100
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: merging discard request in the block layer

On 2011-03-22 20:47, Christoph Hellwig wrote:
> 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

That would seem to indicate a bug in the merging logic instead.

> 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;
>  
>  	/*

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?


-- 
Jens Axboe

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