[PATCH 3.13.y-ckt 103/103] dm cache: fix problematic dual use of a single migration count variable

Kamal Mostafa
2015-02-19T00:35:40Z

From: Joe Thornber <ejt@redhat.com>

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: Kamal Mostafa <kamal@canonical.com>
---
drivers/md/dm-cache-target.c | 89 +++++++++++++++++++++++++-------------------
1 file changed, 50 insertions(+), 39 deletions(-)

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 4a1a491..24e4ae3 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -216,7 +216,13 @@ 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;
@@ -253,7 +259,6 @@ 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;
@@ -344,10 +349,31 @@ 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;
}
@@ -376,7 +402,7 @@ 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)
@@ -803,24 +829,14 @@ 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,
@@ -843,11 +859,10 @@ 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)
@@ -872,7 +887,7 @@ 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)
@@ -883,7 +898,7 @@ 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) {
@@ -893,14 +908,14 @@ 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;
}
}
@@ -933,7 +948,7 @@ 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 {
@@ -948,7 +963,7 @@ 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);
}
}

@@ -1158,7 +1173,7 @@ 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);
}

@@ -1181,7 +1196,7 @@ 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);
}

@@ -1207,7 +1222,7 @@ 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);
}

@@ -1234,7 +1249,7 @@ 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);
}

@@ -1295,7 +1310,7 @@ 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;
}
@@ -1650,7 +1665,7 @@ 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)
@@ -1761,9 +1776,6 @@ 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);

@@ -2271,7 +2283,8 @@ 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);
@@ -2331,8 +2344,6 @@ 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;
--
1.9.1