dm snapshot: track snapshot reads

Whenever a snapshot read gets mapped through to the origin, track it in
a per-snapshot hash table indexed by chunk number, using memory allocated
from a new per-snapshot mempool.

We need to track these reads to avoid race conditions which will be fixed
by patches that follow.

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

2 files changed, 106 insertions, 10 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 1ba8a47d61b1..de302702ab3e 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -40,6 +40,11 @@
  */
 #define SNAPSHOT_PAGES (((1UL << 20) >> PAGE_SHIFT) ? : 1)
 
+/*
+ * The size of the mempool used to track chunks in use.
+ */
+#define MIN_IOS 256
+
 static struct workqueue_struct *ksnapd;
 static void flush_queued_bios(struct work_struct *work);
 
@@ -93,6 +98,42 @@ static struct kmem_cache *exception_cache;
 static struct kmem_cache *pending_cache;
 static mempool_t *pending_pool;
 
+struct dm_snap_tracked_chunk {
+        struct hlist_node node;
+        chunk_t chunk;
+};
+
+static struct kmem_cache *tracked_chunk_cache;
+
+static struct dm_snap_tracked_chunk *track_chunk(struct dm_snapshot *s,
+                                                 chunk_t chunk)
+{
+        struct dm_snap_tracked_chunk *c = mempool_alloc(s->tracked_chunk_pool,
+                                                        GFP_NOIO);
+        unsigned long flags;
+
+        c->chunk = chunk;
+
+        spin_lock_irqsave(&s->tracked_chunk_lock, flags);
+        hlist_add_head(&c->node,
+                       &s->tracked_chunk_hash[DM_TRACKED_CHUNK_HASH(chunk)]);
+        spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);
+
+        return c;
+}
+
+static void stop_tracking_chunk(struct dm_snapshot *s,
+                                struct dm_snap_tracked_chunk *c)
+{
+        unsigned long flags;
+
+        spin_lock_irqsave(&s->tracked_chunk_lock, flags);
+        hlist_del(&c->node);
+        spin_unlock_irqrestore(&s->tracked_chunk_lock, flags);
+
+        mempool_free(c, s->tracked_chunk_pool);
+}
+
 /*
  * One of these per registered origin, held in the snapshot_origins hash
  */
@@ -482,6 +523,7 @@ static int set_chunk_size(struct dm_snapshot *s, const char *chunk_size_arg,
 static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 {
         struct dm_snapshot *s;
+        int i;
         int r = -EINVAL;
         char persistent;
         char *origin_path;
@@ -564,11 +606,24 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
                 goto bad5;
         }
 
+        s->tracked_chunk_pool = mempool_create_slab_pool(MIN_IOS,
+                                                         tracked_chunk_cache);
+        if (!s->tracked_chunk_pool) {
+                ti->error = "Could not allocate tracked_chunk mempool for "
+                            "tracking reads";
+                goto bad6;
+        }
+
+        for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
+                INIT_HLIST_HEAD(&s->tracked_chunk_hash[i]);
+
+        spin_lock_init(&s->tracked_chunk_lock);
+
         /* Metadata must only be loaded into one table at once */
         r = s->store.read_metadata(&s->store);
         if (r < 0) {
                 ti->error = "Failed to read snapshot metadata";
-                goto bad6;
+                goto bad_load_and_register;
         } else if (r > 0) {
                 s->valid = 0;
                 DMWARN("Snapshot is marked invalid.");
@@ -582,7 +637,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
         if (register_snapshot(s)) {
                 r = -EINVAL;
                 ti->error = "Cannot register snapshot origin";
-                goto bad6;
+                goto bad_load_and_register;
         }
 
         ti->private = s;
@@ -590,6 +645,9 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
         return 0;
 
+ bad_load_and_register:
+        mempool_destroy(s->tracked_chunk_pool);
+
  bad6:
         dm_kcopyd_client_destroy(s->kcopyd_client);
 
@@ -624,6 +682,9 @@ static void __free_exceptions(struct dm_snapshot *s)
 
 static void snapshot_dtr(struct dm_target *ti)
 {
+#ifdef CONFIG_DM_DEBUG
+        int i;
+#endif
         struct dm_snapshot *s = ti->private;
 
         flush_workqueue(ksnapd);
@@ -632,6 +693,13 @@ static void snapshot_dtr(struct dm_target *ti)
         /* After this returns there can be no new kcopyd jobs. */
         unregister_snapshot(s);
 
+#ifdef CONFIG_DM_DEBUG
+        for (i = 0; i < DM_TRACKED_CHUNK_HASH_SIZE; i++)
+                BUG_ON(!hlist_empty(&s->tracked_chunk_hash[i]));
+#endif
+
+        mempool_destroy(s->tracked_chunk_pool);
+
         __free_exceptions(s);
 
         dm_put_device(ti, s->origin);
@@ -974,14 +1042,10 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
                         start_copy(pe);
                         goto out;
                 }
-        } else
-                /*
-                 * FIXME: this read path scares me because we
-                 * always use the origin when we have a pending
-                 * exception.  However I can't think of a
-                 * situation where this is wrong - ejt.
-                 */
+        } else {
                 bio->bi_bdev = s->origin->bdev;
+                map_context->ptr = track_chunk(s, chunk);
+        }
 
  out_unlock:
         up_write(&s->lock);
@@ -989,6 +1053,18 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
         return r;
 }
 
+static int snapshot_end_io(struct dm_target *ti, struct bio *bio,
+                           int error, union map_info *map_context)
+{
+        struct dm_snapshot *s = ti->private;
+        struct dm_snap_tracked_chunk *c = map_context->ptr;
+
+        if (c)
+                stop_tracking_chunk(s, c);
+
+        return 0;
+}
+
 static void snapshot_resume(struct dm_target *ti)
 {
         struct dm_snapshot *s = ti->private;
@@ -1266,6 +1342,7 @@ static struct target_type snapshot_target = {
         .ctr     = snapshot_ctr,
         .dtr     = snapshot_dtr,
         .map     = snapshot_map,
+        .end_io  = snapshot_end_io,
         .resume  = snapshot_resume,
         .status  = snapshot_status,
 };
@@ -1306,11 +1383,18 @@ static int __init dm_snapshot_init(void)
                 goto bad4;
         }
 
+        tracked_chunk_cache = KMEM_CACHE(dm_snap_tracked_chunk, 0);
+        if (!tracked_chunk_cache) {
+                DMERR("Couldn't create cache to track chunks in use.");
+                r = -ENOMEM;
+                goto bad5;
+        }
+
         pending_pool = mempool_create_slab_pool(128, pending_cache);
         if (!pending_pool) {
                 DMERR("Couldn't create pending pool.");
                 r = -ENOMEM;
-                goto bad5;
+                goto bad_pending_pool;
         }
 
         ksnapd = create_singlethread_workqueue("ksnapd");
@@ -1324,6 +1408,8 @@ static int __init dm_snapshot_init(void)
 
       bad6:
         mempool_destroy(pending_pool);
+      bad_pending_pool:
+        kmem_cache_destroy(tracked_chunk_cache);
       bad5:
         kmem_cache_destroy(pending_cache);
       bad4:
@@ -1355,6 +1441,7 @@ static void __exit dm_snapshot_exit(void)
         mempool_destroy(pending_pool);
         kmem_cache_destroy(pending_cache);
         kmem_cache_destroy(exception_cache);
+        kmem_cache_destroy(tracked_chunk_cache);
 }
 
 /* Module hooks */
diff --git a/drivers/md/dm-snap.h b/drivers/md/dm-snap.h
index 24f9fb73b982..70dc961f40d8 100644
--- a/drivers/md/dm-snap.h
+++ b/drivers/md/dm-snap.h
@@ -130,6 +130,10 @@ struct exception_store {
         void *context;
 };
 
+#define DM_TRACKED_CHUNK_HASH_SIZE      16
+#define DM_TRACKED_CHUNK_HASH(x)        ((unsigned long)(x) & \
+                                         (DM_TRACKED_CHUNK_HASH_SIZE - 1))
+
 struct dm_snapshot {
         struct rw_semaphore lock;
         struct dm_target *ti;
@@ -174,6 +178,11 @@ struct dm_snapshot {
         /* Queue of snapshot writes for ksnapd to flush */
         struct bio_list queued_bios;
         struct work_struct queued_bios_work;
+
+        /* Chunks with outstanding reads */
+        mempool_t *tracked_chunk_pool;
+        spinlock_t tracked_chunk_lock;
+        struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
 };
 
 /*