dm: introduce merge_bvec_fn

Introduce a bvec merge function for device mapper devices
for dynamic size restrictions.

This code ensures the requested biovec lies within a single
target and then calls a target-specific function to check
against any constraints imposed by underlying devices.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index efe969074928..bca448e11878 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -829,6 +829,49 @@ static int __split_bio(struct mapped_device *md, struct bio *bio)
  * CRUD END
  *---------------------------------------------------------------*/
 
+static int dm_merge_bvec(struct request_queue *q,
+                         struct bvec_merge_data *bvm,
+                         struct bio_vec *biovec)
+{
+        struct mapped_device *md = q->queuedata;
+        struct dm_table *map = dm_get_table(md);
+        struct dm_target *ti;
+        sector_t max_sectors;
+        int max_size;
+
+        if (unlikely(!map))
+                return 0;
+
+        ti = dm_table_find_target(map, bvm->bi_sector);
+
+        /*
+         * Find maximum amount of I/O that won't need splitting
+         */
+        max_sectors = min(max_io_len(md, bvm->bi_sector, ti),
+                          (sector_t) BIO_MAX_SECTORS);
+        max_size = (max_sectors << SECTOR_SHIFT) - bvm->bi_size;
+        if (max_size < 0)
+                max_size = 0;
+
+        /*
+         * merge_bvec_fn() returns number of bytes
+         * it can accept at this offset
+         * max is precomputed maximal io size
+         */
+        if (max_size && ti->type->merge)
+                max_size = ti->type->merge(ti, bvm, biovec, max_size);
+
+        /*
+         * Always allow an entire first page
+         */
+        if (max_size <= biovec->bv_len && !(bvm->bi_size >> SECTOR_SHIFT))
+                max_size = biovec->bv_len;
+
+        dm_table_put(map);
+
+        return max_size;
+}
+
 /*
  * The request function that just remaps the bio built up by
  * dm_merge_bvec.
@@ -1032,6 +1075,7 @@ static struct mapped_device *alloc_dev(int minor)
         blk_queue_make_request(md->queue, dm_request);
         blk_queue_bounce_limit(md->queue, BLK_BOUNCE_ANY);
         md->queue->unplug_fn = dm_unplug_all;
+        blk_queue_merge_bvec(md->queue, dm_merge_bvec);
 
         md->io_pool = mempool_create_slab_pool(MIN_IOS, _io_cache);
         if (!md->io_pool)