Re: [PATCH 1/2] block: fix leaks associated with discard request payload

From: Christoph Hellwig
Date: Sun Jun 27 2010 - 05:38:09 EST

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@xxxxxxxxxx> 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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