btrfs: load free space cache asynchronously

While documenting the usage of the commit_root_sem, I noticed that we do
not actually take the commit_root_sem in the case of the free space
cache.  This is problematic because we're supposed to hold that sem
while we're reading the commit roots, which is what we do for the free
space cache.

The reason I did it inline when I originally wrote the code was because
there's the case of unpinning where we need to make sure that the free
space cache is loaded if we're going to use the free space cache.  But
we can accomplish the same thing by simply waiting for the cache to be
loaded.

Rework this code to load the free space cache asynchronously.  This
allows us to greatly cleanup the caching code because now it's all
shared by the various caching methods.  We also are now in a position to
have the commit_root semaphore held while we're loading the free space
cache.  And finally our modification of ->last_byte_to_unpin is removed
because it can be handled in the proper way on commit.

Some care must be taken when replaying the log, when we expect that the
free space cache will be read entirely before we start excluding space
to replay. This could lead to overwriting space during replay.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -424,6 +424,23 @@ int btrfs_wait_block_group_cache_done(struct btrfs_block_group *cache)
 	return ret;
 }
 
+static bool space_cache_v1_done(struct btrfs_block_group *cache)
+{
+	bool ret;
+
+	spin_lock(&cache->lock);
+	ret = cache->cached != BTRFS_CACHE_FAST;
+	spin_unlock(&cache->lock);
+
+	return ret;
+}
+
+void btrfs_wait_space_cache_v1_finished(struct btrfs_block_group *cache,
+				struct btrfs_caching_control *caching_ctl)
+{
+	wait_event(caching_ctl->wait, space_cache_v1_done(cache));
+}
+
 #ifdef CONFIG_BTRFS_DEBUG
 static void fragment_free_space(struct btrfs_block_group *block_group)
 {
@@ -639,11 +656,28 @@ static noinline void caching_thread(struct btrfs_work *work)
 	mutex_lock(&caching_ctl->mutex);
 	down_read(&fs_info->commit_root_sem);
 
+	if (btrfs_test_opt(fs_info, SPACE_CACHE)) {
+		ret = load_free_space_cache(block_group);
+		if (ret == 1) {
+			ret = 0;
+			goto done;
+		}
+
+		/*
+		 * We failed to load the space cache, set ourselves to
+		 * CACHE_STARTED and carry on.
+		 */
+		spin_lock(&block_group->lock);
+		block_group->cached = BTRFS_CACHE_STARTED;
+		spin_unlock(&block_group->lock);
+		wake_up(&caching_ctl->wait);
+	}
+
 	if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE))
 		ret = load_free_space_tree(caching_ctl);
 	else
 		ret = load_extent_tree_free(caching_ctl);
-
+done:
 	spin_lock(&block_group->lock);
 	block_group->caching_ctl = NULL;
 	block_group->cached = ret ? BTRFS_CACHE_ERROR : BTRFS_CACHE_FINISHED;
@@ -679,7 +713,7 @@ int btrfs_cache_block_group(struct btrfs_block_group *cache, int load_cache_only
 {
 	DEFINE_WAIT(wait);
 	struct btrfs_fs_info *fs_info = cache->fs_info;
-	struct btrfs_caching_control *caching_ctl;
+	struct btrfs_caching_control *caching_ctl = NULL;
 	int ret = 0;
 
 	caching_ctl = kzalloc(sizeof(*caching_ctl), GFP_NOFS);
@@ -691,83 +725,27 @@ int btrfs_cache_block_group(struct btrfs_block_group *cache, int load_cache_only
 	init_waitqueue_head(&caching_ctl->wait);
 	caching_ctl->block_group = cache;
 	caching_ctl->progress = cache->start;
-	refcount_set(&caching_ctl->count, 1);
+	refcount_set(&caching_ctl->count, 2);
 	btrfs_init_work(&caching_ctl->work, caching_thread, NULL, NULL);
 
 	spin_lock(&cache->lock);
 	if (cache->cached != BTRFS_CACHE_NO) {
-		spin_unlock(&cache->lock);
 		kfree(caching_ctl);
-		return 0;
+
+		caching_ctl = cache->caching_ctl;
+		if (caching_ctl)
+			refcount_inc(&caching_ctl->count);
+		spin_unlock(&cache->lock);
+		goto out;
 	}
 	WARN_ON(cache->caching_ctl);
 	cache->caching_ctl = caching_ctl;
+	if (btrfs_test_opt(fs_info, SPACE_CACHE))
 		cache->cached = BTRFS_CACHE_FAST;
-	spin_unlock(&cache->lock);
-
-	if (btrfs_test_opt(fs_info, SPACE_CACHE)) {
-		mutex_lock(&caching_ctl->mutex);
-		ret = load_free_space_cache(cache);
-
-		spin_lock(&cache->lock);
-		if (ret == 1) {
-			cache->caching_ctl = NULL;
-			cache->cached = BTRFS_CACHE_FINISHED;
-			cache->last_byte_to_unpin = (u64)-1;
-			caching_ctl->progress = (u64)-1;
-		} else {
-			if (load_cache_only) {
-				cache->caching_ctl = NULL;
-				cache->cached = BTRFS_CACHE_NO;
-			} else {
-				cache->cached = BTRFS_CACHE_STARTED;
-				cache->has_caching_ctl = 1;
-			}
-		}
-		spin_unlock(&cache->lock);
-#ifdef CONFIG_BTRFS_DEBUG
-		if (ret == 1 &&
-		    btrfs_should_fragment_free_space(cache)) {
-			u64 bytes_used;
-
-			spin_lock(&cache->space_info->lock);
-			spin_lock(&cache->lock);
-			bytes_used = cache->length - cache->used;
-			cache->space_info->bytes_used += bytes_used >> 1;
-			spin_unlock(&cache->lock);
-			spin_unlock(&cache->space_info->lock);
-			fragment_free_space(cache);
-		}
-#endif
-		mutex_unlock(&caching_ctl->mutex);
-
-		wake_up(&caching_ctl->wait);
-		if (ret == 1) {
-			btrfs_put_caching_control(caching_ctl);
-			btrfs_free_excluded_extents(cache);
-			return 0;
-		}
-	} else {
-		/*
-		 * We're either using the free space tree or no caching at all.
-		 * Set cached to the appropriate value and wakeup any waiters.
-		 */
-		spin_lock(&cache->lock);
-		if (load_cache_only) {
-			cache->caching_ctl = NULL;
-			cache->cached = BTRFS_CACHE_NO;
-		} else {
+	else
 		cache->cached = BTRFS_CACHE_STARTED;
 	cache->has_caching_ctl = 1;
-		}
 	spin_unlock(&cache->lock);
-		wake_up(&caching_ctl->wait);
-	}
-
-	if (load_cache_only) {
-		btrfs_put_caching_control(caching_ctl);
-		return 0;
-	}
 
 	down_write(&fs_info->commit_root_sem);
 	refcount_inc(&caching_ctl->count);
@@ -777,6 +755,11 @@ int btrfs_cache_block_group(struct btrfs_block_group *cache, int load_cache_only
 	btrfs_get_block_group(cache);
 
 	btrfs_queue_work(fs_info->caching_workers, &caching_ctl->work);
+out:
+	if (load_cache_only && caching_ctl)
+		btrfs_wait_space_cache_v1_finished(cache, caching_ctl);
+	if (caching_ctl)
+		btrfs_put_caching_control(caching_ctl);
 
 	return ret;
 }

fs/btrfs/block-group.h

@@ -268,6 +268,8 @@ void check_system_chunk(struct btrfs_trans_handle *trans, const u64 type);
 u64 btrfs_get_alloc_profile(struct btrfs_fs_info *fs_info, u64 orig_flags);
 void btrfs_put_block_group_cache(struct btrfs_fs_info *info);
 int btrfs_free_block_groups(struct btrfs_fs_info *info);
+void btrfs_wait_space_cache_v1_finished(struct btrfs_block_group *cache,
+				struct btrfs_caching_control *caching_ctl);
 
 static inline u64 btrfs_data_alloc_profile(struct btrfs_fs_info *fs_info)
 {

fs/btrfs/extent-tree.c

@@ -2641,6 +2641,11 @@ static int __exclude_logged_extent(struct btrfs_fs_info *fs_info,
 		BUG_ON(!btrfs_block_group_done(block_group));
 		ret = btrfs_remove_free_space(block_group, start, num_bytes);
 	} else {
+		/*
+		 * We must wait for v1 caching to finish, otherwise we may not
+		 * remove our space.
+		 */
+		btrfs_wait_space_cache_v1_finished(block_group, caching_ctl);
 		mutex_lock(&caching_ctl->mutex);
 
 		if (start >= caching_ctl->progress) {