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

commit a59db67656021fa212e9b95a583f13c34eb67cd9 upstream.

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>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 843af26..1771845 100644 (file)
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -146,7 +146,13 @@ 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_ack;
@@ -182,7 +188,6 @@ 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;
@@ -265,10 +270,31 @@ 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;
        }
@@ -297,7 +323,7 @@ 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)
@@ -708,24 +734,14 @@ 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,
@@ -748,11 +764,10 @@ 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)
@@ -777,7 +792,7 @@ static void migration_failure(struct dm_cache_migration *mg)
                cell_defer(cache, mg->new_ocell, 1);
        }
 
-       cleanup_migration(mg);
+       free_io_migration(mg);
 }
 
 static void migration_success_pre_commit(struct dm_cache_migration *mg)
@@ -788,7 +803,7 @@ 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) {
@@ -798,14 +813,14 @@ 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;
                }
        }
@@ -836,12 +851,12 @@ static void migration_success_post_commit(struct dm_cache_migration *mg)
                        spin_unlock_irqrestore(&cache->lock, flags);
 
                } else
-                       cleanup_migration(mg);
+                       free_io_migration(mg);
 
        } else {
                clear_dirty(cache, mg->new_oblock, mg->cblock);
                cell_defer(cache, mg->new_ocell, true);
-               cleanup_migration(mg);
+               free_io_migration(mg);
        }
 }
 
@@ -1002,7 +1017,7 @@ 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);
 }
 
@@ -1023,7 +1038,7 @@ 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);
 }
 
@@ -1047,7 +1062,7 @@ 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);
 }
 
@@ -1108,7 +1123,7 @@ static void process_discard_bio(struct cache *cache, struct bio *bio)
 
 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;
 }
@@ -1399,7 +1414,7 @@ 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)
@@ -1508,9 +1523,6 @@ 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);
 
@@ -1998,7 +2010,8 @@ 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);
@@ -2057,8 +2070,6 @@ 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->quiescing = false;