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

[Mikulas Patocka]

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.

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);
-
 			cell_release_singleton(cell, bio);
 			cell_release_singleton(cell2, bio);
 			remap_and_issue(tc, bio, lookup_result.block);
@@ -2506,7 +2499,8 @@ static void set_discard_limits(struct po
 
 	/*
 	 * This is just a hint, and not enforced.  We have to cope with
-	 * bios that overlap 2 blocks.
+	 * bios 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;
@@ -2648,6 +2642,8 @@ static int thin_ctr(struct dm_target *ti
 	if (tc->pool->pf.discard_enabled) {
 		ti->discards_supported = 1;
 		ti->num_discard_requests = 1;
+		/* Discard requests must be split on a chunk boundary */
+		ti->split_discard_requests = 1;
 	}
 
 	dm_put(pool_md);