dm cache: fix problematic dual use of a single migration count variable

Introduce a new variable to count the number of allocated migration
structures.  The existing variable cache->nr_migrations became
overloaded.  It was used to:

 i) track of the number of migrations in flight for the purposes of
    quiescing during suspend.

 ii) to estimate the amount of background IO occuring.

Recent discard changes meant that REQ_DISCARD bios are processed with
a migration.  Discards are not background IO so nr_migrations was not
incremented.  However this could cause quiescing to complete early.

(i) is now handled with a new variable cache->nr_allocated_migrations.
cache->nr_migrations has been renamed cache->nr_io_migrations.
cleanup_migration() is now called free_io_migration(), since it
decrements that variable.

Also, remove the unused cache->next_migration variable that got replaced
with with prealloc_structs a while ago.

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

1 files changed, 50 insertions, 39 deletions

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 1e96d7889f51..e1650539cc2f 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -221,7 +221,13 @@ struct cache {
         struct list_head need_commit_migrations;
         sector_t migration_threshold;
         wait_queue_head_t migration_wait;
-        atomic_t nr_migrations;
+        atomic_t nr_allocated_migrations;
+
+        /*
+         * The number of in flight migrations that are performing
+         * background io. eg, promotion, writeback.
+         */
+        atomic_t nr_io_migrations;
 
         wait_queue_head_t quiescing_wait;
         atomic_t quiescing;
@@ -258,7 +264,6 @@ struct cache {
         struct dm_deferred_set *all_io_ds;
 
         mempool_t *migration_pool;
-        struct dm_cache_migration *next_migration;
 
         struct dm_cache_policy *policy;
         unsigned policy_nr_args;
@@ -350,10 +355,31 @@ static void free_prison_cell(struct cache *cache, struct dm_bio_prison_cell *cel
         dm_bio_prison_free_cell(cache->prison, cell);
 }
 
+static struct dm_cache_migration *alloc_migration(struct cache *cache)
+{
+        struct dm_cache_migration *mg;
+
+        mg = mempool_alloc(cache->migration_pool, GFP_NOWAIT);
+        if (mg) {
+                mg->cache = cache;
+                atomic_inc(&mg->cache->nr_allocated_migrations);
+        }
+
+        return mg;
+}
+
+static void free_migration(struct dm_cache_migration *mg)
+{
+        if (atomic_dec_and_test(&mg->cache->nr_allocated_migrations))
+                wake_up(&mg->cache->migration_wait);
+
+        mempool_free(mg, mg->cache->migration_pool);
+}
+
 static int prealloc_data_structs(struct cache *cache, struct prealloc *p)
 {
         if (!p->mg) {
-                p->mg = mempool_alloc(cache->migration_pool, GFP_NOWAIT);
+                p->mg = alloc_migration(cache);
                 if (!p->mg)
                         return -ENOMEM;
         }
@@ -382,7 +408,7 @@ static void prealloc_free_structs(struct cache *cache, struct prealloc *p)
                 free_prison_cell(cache, p->cell1);
 
         if (p->mg)
-                mempool_free(p->mg, cache->migration_pool);
+                free_migration(p->mg);
 }
 
 static struct dm_cache_migration *prealloc_get_migration(struct prealloc *p)
@@ -854,24 +880,14 @@ static void remap_to_origin_then_cache(struct cache *cache, struct bio *bio,
  * Migration covers moving data from the origin device to the cache, or
  * vice versa.
  *--------------------------------------------------------------*/
-static void free_migration(struct dm_cache_migration *mg)
-{
-        mempool_free(mg, mg->cache->migration_pool);
-}
-
-static void inc_nr_migrations(struct cache *cache)
+static void inc_io_migrations(struct cache *cache)
 {
-        atomic_inc(&cache->nr_migrations);
+        atomic_inc(&cache->nr_io_migrations);
 }
 
-static void dec_nr_migrations(struct cache *cache)
+static void dec_io_migrations(struct cache *cache)
 {
-        atomic_dec(&cache->nr_migrations);
-
-        /*
-         * Wake the worker in case we're suspending the target.
-         */
-        wake_up(&cache->migration_wait);
+        atomic_dec(&cache->nr_io_migrations);
 }
 
 static void __cell_defer(struct cache *cache, struct dm_bio_prison_cell *cell,
@@ -894,11 +910,10 @@ static void cell_defer(struct cache *cache, struct dm_bio_prison_cell *cell,
         wake_worker(cache);
 }
 
-static void cleanup_migration(struct dm_cache_migration *mg)
+static void free_io_migration(struct dm_cache_migration *mg)
 {
-        struct cache *cache = mg->cache;
+        dec_io_migrations(mg->cache);
         free_migration(mg);
-        dec_nr_migrations(cache);
 }
 
 static void migration_failure(struct dm_cache_migration *mg)
@@ -923,7 +938,7 @@ static void migration_failure(struct dm_cache_migration *mg)
                 cell_defer(cache, mg->new_ocell, true);
         }
 
-        cleanup_migration(mg);
+        free_io_migration(mg);
 }
 
 static void migration_success_pre_commit(struct dm_cache_migration *mg)
@@ -934,7 +949,7 @@ static void migration_success_pre_commit(struct dm_cache_migration *mg)
         if (mg->writeback) {
                 clear_dirty(cache, mg->old_oblock, mg->cblock);
                 cell_defer(cache, mg->old_ocell, false);
-                cleanup_migration(mg);
+                free_io_migration(mg);
                 return;
 
         } else if (mg->demote) {
@@ -944,14 +959,14 @@ static void migration_success_pre_commit(struct dm_cache_migration *mg)
                                              mg->old_oblock);
                         if (mg->promote)
                                 cell_defer(cache, mg->new_ocell, true);
-                        cleanup_migration(mg);
+                        free_io_migration(mg);
                         return;
                 }
         } else {
                 if (dm_cache_insert_mapping(cache->cmd, mg->cblock, mg->new_oblock)) {
                         DMWARN_LIMIT("promotion failed; couldn't update on disk metadata");
                         policy_remove_mapping(cache->policy, mg->new_oblock);
-                        cleanup_migration(mg);
+                        free_io_migration(mg);
                         return;
                 }
         }
@@ -984,7 +999,7 @@ static void migration_success_post_commit(struct dm_cache_migration *mg)
                 } else {
                         if (mg->invalidate)
                                 policy_remove_mapping(cache->policy, mg->old_oblock);
-                        cleanup_migration(mg);
+                        free_io_migration(mg);
                 }
 
         } else {
@@ -999,7 +1014,7 @@ static void migration_success_post_commit(struct dm_cache_migration *mg)
                         bio_endio(mg->new_ocell->holder, 0);
                         cell_defer(cache, mg->new_ocell, false);
                 }
-                cleanup_migration(mg);
+                free_io_migration(mg);
         }
 }
 
@@ -1251,7 +1266,7 @@ static void promote(struct cache *cache, struct prealloc *structs,
         mg->new_ocell = cell;
         mg->start_jiffies = jiffies;
 
-        inc_nr_migrations(cache);
+        inc_io_migrations(cache);
         quiesce_migration(mg);
 }
 
@@ -1275,7 +1290,7 @@ static void writeback(struct cache *cache, struct prealloc *structs,
         mg->new_ocell = NULL;
         mg->start_jiffies = jiffies;
 
-        inc_nr_migrations(cache);
+        inc_io_migrations(cache);
         quiesce_migration(mg);
 }
 
@@ -1302,7 +1317,7 @@ static void demote_then_promote(struct cache *cache, struct prealloc *structs,
         mg->new_ocell = new_ocell;
         mg->start_jiffies = jiffies;
 
-        inc_nr_migrations(cache);
+        inc_io_migrations(cache);
         quiesce_migration(mg);
 }
 
@@ -1330,7 +1345,7 @@ static void invalidate(struct cache *cache, struct prealloc *structs,
         mg->new_ocell = NULL;
         mg->start_jiffies = jiffies;
 
-        inc_nr_migrations(cache);
+        inc_io_migrations(cache);
         quiesce_migration(mg);
 }
 
@@ -1412,7 +1427,7 @@ static void process_discard_bio(struct cache *cache, struct prealloc *structs,
 
 static bool spare_migration_bandwidth(struct cache *cache)
 {
-        sector_t current_volume = (atomic_read(&cache->nr_migrations) + 1) *
+        sector_t current_volume = (atomic_read(&cache->nr_io_migrations) + 1) *
                 cache->sectors_per_block;
         return current_volume < cache->migration_threshold;
 }
@@ -1764,7 +1779,7 @@ static void stop_quiescing(struct cache *cache)
 
 static void wait_for_migrations(struct cache *cache)
 {
-        wait_event(cache->migration_wait, !atomic_read(&cache->nr_migrations));
+        wait_event(cache->migration_wait, !atomic_read(&cache->nr_allocated_migrations));
 }
 
 static void stop_worker(struct cache *cache)
@@ -1876,9 +1891,6 @@ static void destroy(struct cache *cache)
 {
         unsigned i;
 
-        if (cache->next_migration)
-                mempool_free(cache->next_migration, cache->migration_pool);
-
         if (cache->migration_pool)
                 mempool_destroy(cache->migration_pool);
 
@@ -2424,7 +2436,8 @@ static int cache_create(struct cache_args *ca, struct cache **result)
         INIT_LIST_HEAD(&cache->quiesced_migrations);
         INIT_LIST_HEAD(&cache->completed_migrations);
         INIT_LIST_HEAD(&cache->need_commit_migrations);
-        atomic_set(&cache->nr_migrations, 0);
+        atomic_set(&cache->nr_allocated_migrations, 0);
+        atomic_set(&cache->nr_io_migrations, 0);
         init_waitqueue_head(&cache->migration_wait);
 
         init_waitqueue_head(&cache->quiescing_wait);
@@ -2487,8 +2500,6 @@ static int cache_create(struct cache_args *ca, struct cache **result)
                 goto bad;
         }
 
-        cache->next_migration = NULL;
-
         cache->need_tick_bio = true;
         cache->sized = false;
         cache->invalidate = false;