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