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From: Andrew Morton on 23 Jun 2010 15:50
On Wed, 23 Jun 2010 11:24:51 +0800
Tao Ma <tao.ma(a)oracle.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(a)kernel.dk>
> Signed-off-by: Tao Ma <tao.ma(a)oracle.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. That patch is in
linux-next now, but I expect it will be taken out again (quickly,
please) because Mike has found a number of problems with it.

Your patch looks like a decent temporary fix for mainline, and a
permanent one for -stable.
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