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

[Mikulas Patocka]

On Tue, 17 Jul 2012, Mike Snitzer wrote:

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

The patch refers to the patchset that will be sent out for the next 
kernel: http://people.redhat.com/agk/patches/linux/editing/series.html

In the current 3.5-rc code unaligned discard is partially ignored. In the 
patchset it causes wrong data to be discarded.

> > > 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 at 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

Because the code can't handle large discards --- it trims them to a block 
boundary and sends the trimmed request to the first chunk.

> 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.

When Joe implements this, ti->split_discard_requests could be cleared.

Mikulas