Re: [PATCH 2/3] dm-thin: fix discard support

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jul 17 2012 at  9:26am -0400,
Vivek Goyal <vgoyal@redhat.com> wrote:

> On Mon, Jul 16, 2012 at 02:35:18PM -0400, Mikulas Patocka wrote:
> > dm-thin: fix discard support
> > 
> > There is a bug in dm_thin regarding processing discards.
> > When dm-thin receives a discard request with size equal to block size
> > that is not aligned on block size boundary, io_overlaps_block returns
> > true, process_discard treats this discard as a full block discard,
>   ^^^^
> > deletes the full block - the result is that some data that shouldn't be
> > discarded are discarded.
> 
> Looking at io_overlaps_block(), it looks like it will return false (and
> not true) for bios which are not aligned to block size boundary.
> 
> static int io_overlaps_block(struct pool *pool, struct bio *bio)
> {
>         return !(bio->bi_sector & pool->offset_mask) &&
>                 (bio->bi_size == (pool->sectors_per_block << SECTOR_SHIFT));
> 
> }
> 
> Hence for block which crosses block size boundary, we should be sending
> discard down for partial block as per the current code and no harm should
> be done?

Right, not sure why Mikulas read that as it'd return true.
 
> > This patch sets the variable "ti->split_discard_requests", so that
> > device mapper core splits discard requests on a block boundary.
> > 
> > Consequently, a discard request that spans multiple blocks is never sent
> > to dm-thin. The patch also removes some code in process_discard that
> > deals with discards that span multiple blocks.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > ---
> >  drivers/md/dm-thin.c |   18 +++++++-----------
> >  1 file changed, 7 insertions(+), 11 deletions(-)
> > 
> > Index: linux-3.5-rc6-fast/drivers/md/dm-thin.c
> > ===================================================================
> > --- linux-3.5-rc6-fast.orig/drivers/md/dm-thin.c	2012-07-16 18:46:18.000000000 +0200
> > +++ linux-3.5-rc6-fast/drivers/md/dm-thin.c	2012-07-16 20:07:19.000000000 +0200
> > @@ -1246,17 +1246,10 @@ static void process_discard(struct thin_
> >  			}
> >  		} else {
> >  			/*
> > -			 * This path is hit if people are ignoring
> > -			 * limits->discard_granularity.  It ignores any
> > -			 * part of the discard that is in a subsequent
> > -			 * block.
> > +			 * The dm makes sure that the discard doesn't span
> > +			 * a block boundary. So we submit the discard
> > +			 * to the appropriate block.
> >  			 */
> > -			sector_t offset = pool->sectors_per_block_shift >= 0 ?
> > -			      bio->bi_sector & (pool->sectors_per_block - 1) :
> > -			      bio->bi_sector - block * pool->sectors_per_block;
> > -			unsigned remaining = (pool->sectors_per_block - offset) << SECTOR_SHIFT;
> > -			bio->bi_size = min(bio->bi_size, remaining);
> > -
> 
> So previous code will also send down partial block discard and this code
> will also send down partial discard. So nothing has changed from
> functionality point of view?

The change is the bit that you trimmed:

ti->split_discard_requests = 1;

That will restrict the size of the discard to be on a blocksize
boundary.

But I'm really not sure we want to impose such small discards -- though
the current thinp code does have a problem with discards that are too
large (I need to dig up specifics that Joe conveyed to me a few weeks
back; I was asking: "why cannot the thinp device have discard limits
that match the underlying data device's discard limits?").

<snitm> why not just rely on the max of the underlying device?
<ejt> we have to quiesce the blocks we'e about to discard
<snitm> e.g. remove the explicit override for max_bytes in set_discard_limits?
<ejt> no, it's not simple at all.
<ejt> have to be very careful we can service the discard in bounded memory
<snitm> so you don't think thinp can handle processing what the hardware can?
<ejt> not yet; I'd need to change the bio_prison to be able to lock
ranges of blocks, not just single ones.  And add a btree_trim method to
prune a btree.