dm: fix casting bug in dm_merge_bvec()

dm_merge_bvec() was originally added in f6fccb ("dm: introduce
merge_bvec_fn").  In that commit a value in sectors is converted to
bytes using << 9, and then assigned to an int.  This code made
assumptions about the value of BIO_MAX_SECTORS.

A later commit 148e51 ("dm: improve documentation and code clarity in
dm_merge_bvec") was meant to have no functional change but it removed
the use of BIO_MAX_SECTORS in favor of using queue_max_sectors().  At
this point the cast from sector_t to int resulted in a zero value.  The
fallout being dm_merge_bvec() would only allow a single page to be added
to a bio.

This interim fix is minimal for the benefit of stable@ because the more
comprehensive cleanup of passing a sector_t to all DM targets' merge
function will impact quite a few DM targets.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.19+

1 files changed, 12 insertions, 5 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index e24069aaeb18..2caf492890d6 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1723,8 +1723,7 @@ static int dm_merge_bvec(struct request_queue *q,
         struct mapped_device *md = q->queuedata;
         struct dm_table *map = dm_get_live_table_fast(md);
         struct dm_target *ti;
-        sector_t max_sectors;
-        int max_size = 0;
+        sector_t max_sectors, max_size = 0;
 
         if (unlikely(!map))
                 goto out;
@@ -1739,8 +1738,16 @@ static int dm_merge_bvec(struct request_queue *q,
         max_sectors = min(max_io_len(bvm->bi_sector, ti),
                           (sector_t) queue_max_sectors(q));
         max_size = (max_sectors << SECTOR_SHIFT) - bvm->bi_size;
-        if (unlikely(max_size < 0)) /* this shouldn't _ever_ happen */
-                max_size = 0;
+
+        /*
+         * FIXME: this stop-gap fix _must_ be cleaned up (by passing a sector_t
+         * to the targets' merge function since it holds sectors not bytes).
+         * Just doing this as an interim fix for stable@ because the more
+         * comprehensive cleanup of switching to sector_t will impact every
+         * DM target that implements a ->merge hook.
+         */
+        if (max_size > INT_MAX)
+                max_size = INT_MAX;
 
         /*
          * merge_bvec_fn() returns number of bytes
@@ -1748,7 +1755,7 @@ static int dm_merge_bvec(struct request_queue *q,
          * max is precomputed maximal io size
          */
         if (max_size && ti->type->merge)
-                max_size = ti->type->merge(ti, bvm, biovec, max_size);
+                max_size = ti->type->merge(ti, bvm, biovec, (int) max_size);
         /*
          * If the target doesn't support merge method and some of the devices
          * provided their merge_bvec method (we know this by looking for the