Re: [PATCH v7 4/5] dm-thin: Add REQ_OP_PROVISION support

[Mike Snitzer]

On Fri, May 19 2023 at 11:23P -0400,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:

> On Thu, May 18 2023 at  6:33P -0400,
> Sarthak Kukreti <sarthakkukreti@xxxxxxxxxxxx> wrote:
> 
> > dm-thinpool uses the provision request to provision
> > blocks for a dm-thin device. dm-thinpool currently does not
> > pass through REQ_OP_PROVISION to underlying devices.
> > 
> > For shared blocks, provision requests will break sharing and copy the
> > contents of the entire block. Additionally, if 'skip_block_zeroing'
> > is not set, dm-thin will opt to zero out the entire range as a part
> > of provisioning.
> > 
> > Signed-off-by: Sarthak Kukreti <sarthakkukreti@xxxxxxxxxxxx>
> > ---
> >  drivers/md/dm-thin.c | 74 +++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 70 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
> > index 2b13c949bd72..f1b68b558cf0 100644
> > --- a/drivers/md/dm-thin.c
> > +++ b/drivers/md/dm-thin.c
> > @@ -1245,8 +1247,8 @@ static int io_overlaps_block(struct pool *pool, struct bio *bio)
> >  
> >  static int io_overwrites_block(struct pool *pool, struct bio *bio)
> >  {
> > -	return (bio_data_dir(bio) == WRITE) &&
> > -		io_overlaps_block(pool, bio);
> > +	return (bio_data_dir(bio) == WRITE) && io_overlaps_block(pool, bio) &&
> > +	       bio_op(bio) != REQ_OP_PROVISION;
> >  }
> >  
> >  static void save_and_set_endio(struct bio *bio, bio_end_io_t **save,
> > @@ -1394,6 +1396,9 @@ static void schedule_zero(struct thin_c *tc, dm_block_t virt_block,
> >  	m->data_block = data_block;
> >  	m->cell = cell;
> >  
> > +	if (bio && bio_op(bio) == REQ_OP_PROVISION)
> > +		m->bio = bio;
> > +
> >  	/*
> >  	 * If the whole block of data is being overwritten or we are not
> >  	 * zeroing pre-existing data, we can issue the bio immediately.
> 
> This doesn't seem like the best way to address avoiding passdown of
> provision bios (relying on process_prepared_mapping's implementation
> that happens to do the right thing if m->bio set).  Doing so cascades
> into relying on complete_overwrite_bio() happening to _not_ actually
> being specific to "overwrite" bios.
> 
> I don't have a better suggestion yet but will look closer.  Just think
> this needs to be formalized a bit more rather than it happening to
> "just work".
> 
> Cc'ing Joe to see what he thinks too.  This is something we can clean
> up with a follow-on patch though, so not a show-stopper for this
> series.

I haven't circled back to look close enough at this but
REQ_OP_PROVISION bios _are_ being passed down to the thin-pool's
underlying data device.

Brian Foster reported that if he issues a REQ_OP_PROVISION to a thin
device after a snapshot (to break sharing), it'll fail with
-EOPNOTSUPP (response is from bio being passed down to device that
doesn't support it).  I was able to reproduce with:

# fallocate --offset 0 --length 1048576 /dev/test/thin
# lvcreate -n snap --snapshot test/thin

# fallocate --offset 0 --length 1048576 /dev/test/thin
fallocate: fallocate failed: Operation not supported

But reports success when retried:
# fallocate --offset 0 --length 1048576 /dev/test/thin
# echo $?
0

It's somewhat moot in that Joe will be reimplementing handling for
REQ_OP_PROVISION _but_ in the meantime it'd be nice to have a version
of this patch that doesn't error (due to passdown of REQ_OP_PROVISION)
when breaking sharing.  Primarily so the XFS guys (Dave and Brian) can
make progress.

I'll take a closer look tomorrow but figured I'd let you know.

Mike