dm: introduce num_flush_requests

Introduce num_flush_requests for a target to set to say how many flush
instructions (empty barriers) it wants to receive.  These are sent by
__clone_and_map_empty_barrier with map_info->flush_request going from 0
to (num_flush_requests - 1).

Old targets without flush support won't receive any flush requests.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

2 files changed, 50 insertions, 0 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 7d9ca709433..badb7519ccc 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -750,6 +750,40 @@ static struct bio *clone_bio(struct bio *bio, sector_t sector,
         return clone;
 }
 
+static void __flush_target(struct clone_info *ci, struct dm_target *ti,
+                          unsigned flush_nr)
+{
+        struct dm_target_io *tio = alloc_tio(ci->md);
+        struct bio *clone;
+
+        tio->io = ci->io;
+        tio->ti = ti;
+
+        memset(&tio->info, 0, sizeof(tio->info));
+        tio->info.flush_request = flush_nr;
+
+        clone = bio_alloc_bioset(GFP_NOIO, 0, ci->md->bs);
+        __bio_clone(clone, ci->bio);
+        clone->bi_destructor = dm_bio_destructor;
+
+        __map_bio(ti, clone, tio);
+}
+
+static int __clone_and_map_empty_barrier(struct clone_info *ci)
+{
+        unsigned target_nr = 0, flush_nr;
+        struct dm_target *ti;
+
+        while ((ti = dm_table_get_target(ci->map, target_nr++)))
+                for (flush_nr = 0; flush_nr < ti->num_flush_requests;
+                     flush_nr++)
+                        __flush_target(ci, ti, flush_nr);
+
+        ci->sector_count = 0;
+
+        return 0;
+}
+
 static int __clone_and_map(struct clone_info *ci)
 {
         struct bio *clone, *bio = ci->bio;
@@ -757,6 +791,9 @@ static int __clone_and_map(struct clone_info *ci)
         sector_t len = 0, max;
         struct dm_target_io *tio;
 
+        if (unlikely(bio_empty_barrier(bio)))
+                return __clone_and_map_empty_barrier(ci);
+
         ti = dm_table_find_target(ci->map, ci->sector);
         if (!dm_target_is_valid(ti))
                 return -EIO;
@@ -877,6 +914,8 @@ static void __split_and_process_bio(struct mapped_device *md, struct bio *bio)
         ci.io->md = md;
         ci.sector = bio->bi_sector;
         ci.sector_count = bio_sectors(bio);
+        if (unlikely(bio_empty_barrier(bio)))
+                ci.sector_count = 1;
         ci.idx = bio->bi_idx;
 
         start_io_acct(ci.io);
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 49c2362977f..fc36a4d0772 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -21,6 +21,7 @@ typedef enum { STATUSTYPE_INFO, STATUSTYPE_TABLE } status_type_t;
 union map_info {
         void *ptr;
         unsigned long long ll;
+        unsigned flush_request;
 };
 
 /*
@@ -168,6 +169,16 @@ struct dm_target {
         sector_t split_io;
 
         /*
+         * A number of zero-length barrier requests that will be submitted
+         * to the target for the purpose of flushing cache.
+         *
+         * The request number will be placed in union map_info->flush_request.
+         * It is a responsibility of the target driver to remap these requests
+         * to the real underlying devices.
+         */
+        unsigned num_flush_requests;
+
+        /*
          * These are automatically filled in by
          * dm_table_get_device.
          */