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    SubjectRe: [PATCH 1/2] block: fix leaks associated with discard request payload
    On Sun, Jun 27, 2010 at 05:49:29PM +0900, FUJITA Tomonori wrote:
    > On Sat, 26 Jun 2010 15:56:50 -0400
    > Mike Snitzer <snitzer@redhat.com> wrote:
    >
    > > Fix leaks introduced via "block: don't allocate a payload for discard
    > > request" commit a1d949f5f44.
    > >
    > > sd_done() is not called for REQ_TYPE_BLOCK_PC commands so cleanup
    > > discard request's payload directly in scsi_finish_command().
    >
    > Instead of adding another discard hack to scsi_finish_command(), how
    > about converting discard to REQ_TYPE_FS request? discard is FS request
    > from the perspective of the block layer. It also fixes a problem that
    > discard isn't retried in the case of UNIT ATTENTION.
    >
    > I think that we can get more cleaner code if we handle discard as
    > normal (fs) request in the block layer (and scsi-ml). We need more
    > changes but this patch is the first step.

    Making discard a REQ_TYPE_FS inside scsi (it already is before entering
    sd_prep_fn) means we'll need to special case it all over the I/O
    submission and completion path. Having the payload length not matching
    the transfer length is something we don't expect for FS requests.

    > index e16185b..9e15c46 100644
    > --- a/block/blk-lib.c
    > +++ b/block/blk-lib.c
    > @@ -20,6 +20,10 @@ static void blkdev_discard_end_io(struct bio *bio, int err)
    > if (bio->bi_private)
    > complete(bio->bi_private);
    >
    > + /* free the page that the lower layer allocated */
    > + if (bio_page(bio))
    > + __free_page(bio_page(bio));
    > +

    This is exactly what this patchkit gets rid off. Having a payload
    page that the caller tracks (previously fully, with this patch only for
    freeing) makes DM's life a lot harder. Remember we don't actually store
    any payload in there before entering sd_prep_fn - it's just that the
    scsi commands implementing discards need some payload - either a sector
    sizes zero filled buffer for WRITE SAME, or an LBA/len encoding inside
    the payload for UNMAP.


    > - rq->cmd_type = REQ_TYPE_BLOCK_PC;
    > + rq->cmd_type = REQ_TYPE_FS;

    No need to set REQ_TYPE_FS here, it's already set up that way.




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