dm thin: split discards on block boundary

This patch sets the variable "ti->split_discard_requests" for the dm thin
target 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>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 7 insertions, 9 deletions

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index f21d318d98f0..828649256902 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -1238,15 +1238,10 @@ static void process_discard(struct thin_c *tc, struct bio *bio)
                         }
                 } 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 core makes sure that the discard doesn't span
+                         * a block boundary.  So we submit the discard of a
+                         * partial block appropriately.
                          */
-                        sector_t offset = 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);
-
                         cell_release_singleton(cell, bio);
                         cell_release_singleton(cell2, bio);
                         if ((!lookup_result.shared) && pool->pf.discard_passdown)
@@ -2509,7 +2504,8 @@ static void set_discard_limits(struct pool *pool, struct queue_limits *limits)
 
         /*
          * This is just a hint, and not enforced.  We have to cope with
-         * bios that overlap 2 blocks.
+         * bios that cover a block partially.  A discard that spans a block
+         * boundary is not sent to this target.
          */
         limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
         limits->discard_zeroes_data = pool->pf.zero_new_blocks;
@@ -2652,6 +2648,8 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
                 ti->discards_supported = 1;
                 ti->num_discard_requests = 1;
                 ti->discard_zeroes_data_unsupported = 1;
+                /* Discard requests must be split on a block boundary */
+                ti->split_discard_requests = 1;
         }
 
         dm_put(pool_md);