linux-kernel - Re: block: Don't count_vm_events for discard bio in submit

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Date:	Wed, 23 Jun 2010 16:41:19 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tao Ma <tao.ma@...cle.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@....de>, stable@...nel.org
Subject: Re: block: Don't count_vm_events for discard bio in submit_bio.

On Wed, Jun 23 2010 at  3:40pm -0400,
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 23 Jun 2010 11:24:51 +0800
> Tao Ma <tao.ma@...cle.com> wrote:
> 
> > In submit_bio, we count vm events by check READ/WRITE.
> > But actually DISCARD_NOBARRIER also has the WRITE flag set.
> > It looks as if in blkdev_issue_discard, we also add a
> > page as the payload and the bio_has_data check isn't enough.
> > So add another check for discard bio.
> > 
> > Cc: Jens Axboe <axboe@...nel.dk>
> > Signed-off-by: Tao Ma <tao.ma@...cle.com>
> > ---
> >  block/blk-core.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index f84cce4..a725602 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -1586,7 +1586,7 @@ void submit_bio(int rw, struct bio *bio)
> >  	 * If it's a regular read/write or a barrier with data attached,
> >  	 * go through the normal accounting stuff before submission.
> >  	 */
> > -	if (bio_has_data(bio)) {
> > +	if (bio_has_data(bio) && !(rw & BIO_RW_DISCARD)) {
> >  		if (rw & WRITE) {
> >  			count_vm_events(PGPGOUT, count);
> >  		} else {
> 
> Yes, that's a buglet.
> 
> Note that Christoph's "[PATCH, RFC] block: don't allocate a payload for
> discard request" will fix it in a better way.

Oddly, even with Christoph's patch, bio_has_data() is still true once
the discard bio gets to DM.

Figuring out why is on my near-term TODO.

Mike
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