linux-kernel - Re: [PATCH 2/3] xen/blkback: Fix the inhibition to map pages when discarding sector ranges.

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Message-Id: <4E940B99020000780005AA12@nat28.tlf.novell.com>
Date:	Tue, 11 Oct 2011 08:25:45 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Dong Yang Li" <lidongyang@...e.com>
Cc:	<hch@...radead.org>, <xen-devel@...ts.xensource.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] xen/blkback: Fix the inhibition to map pages
	 when discarding sector ranges.

>>> On 10.10.11 at 17:28, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> The 'operation' parameters are the ones provided to the bio layer while
> the req->operation are the ones passed in between the backend and
> frontend. We used the wrong 'operation' value to squash the
> call to map pages when processing the discard operation resulting
> in mapping the pages unnecessarily.
> 
> CC: Li Dongyang <lidongyang@...ell.com>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  drivers/block/xen-blkback/blkback.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/blkback.c 
> b/drivers/block/xen-blkback/blkback.c
> index 184b133..3da9a40 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -707,7 +707,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
>  	 * the hypercall to unmap the grants - that is all done in
>  	 * xen_blkbk_unmap.
>  	 */
> -	if (operation != BLKIF_OP_DISCARD &&
> +	if (operation != REQ_DISCARD &&

Why is that check necessary in the first place? xen_blkbk_map() doesn't
do any harm when req->nr_segments is zero (as could also be the case
on WRITE_FLUSH ones).

Jan

>  			xen_blkbk_map(req, pending_req, seg))
>  		goto fail_flush;
>  




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