Btrfs: fix total_bytes_pinned counter

From: Omar Sandoval <[email protected]>

There are a few places where we pass in a negative num_bytes, so make it signed for clarity. Also move it up in the file since later patches will need it there.

Signed-off-by Omar Sandoval <[email protected]>

From: Omar Sandoval <[email protected]>

There are a few places where we pass in a negative num_bytes, so make it signed for clarity. Also move it up in the file since later patches will need it there.

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/extent-tree.c | 41 ++++++++++++++++++++---------------------
 1 file changed, 20 insertions(+), 21 deletions(-)

fs/btrfs/extent-tree.c +20 -21

index e390451c72e6..7c01b4e9e3b6 100644
@@ -766,6 +766,26 @@ static struct btrfs_space_info *__find_space_info(struct btrfs_fs_info *info,
 	return NULL;
 }
 
+static void add_pinned_bytes(struct btrfs_fs_info *fs_info, s64 num_bytes,
+			     u64 owner, u64 root_objectid)
+{
+	struct btrfs_space_info *space_info;
+	u64 flags;
+
+	if (owner < BTRFS_FIRST_FREE_OBJECTID) {
+		if (root_objectid == BTRFS_CHUNK_TREE_OBJECTID)
+			flags = BTRFS_BLOCK_GROUP_SYSTEM;
+		else
+			flags = BTRFS_BLOCK_GROUP_METADATA;
+	} else {
+		flags = BTRFS_BLOCK_GROUP_DATA;
+	}
+
+	space_info = __find_space_info(fs_info, flags);
+	BUG_ON(!space_info); /* Logic bug */
+	percpu_counter_add(&space_info->total_bytes_pinned, num_bytes);
+}
+
 /*
  * after adding space to the filesystem, we need to clear the full flags
  * on all the space infos.

@@ -6793,27 +6813,6 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans,
 	return 0;
 }
 
-static void add_pinned_bytes(struct btrfs_fs_info *fs_info, u64 num_bytes,
-			     u64 owner, u64 root_objectid)
-{
-	struct btrfs_space_info *space_info;
-	u64 flags;
-
-	if (owner < BTRFS_FIRST_FREE_OBJECTID) {
-		if (root_objectid == BTRFS_CHUNK_TREE_OBJECTID)
-			flags = BTRFS_BLOCK_GROUP_SYSTEM;
-		else
-			flags = BTRFS_BLOCK_GROUP_METADATA;
-	} else {
-		flags = BTRFS_BLOCK_GROUP_DATA;
-	}
-
-	space_info = __find_space_info(fs_info, flags);
-	BUG_ON(!space_info); /* Logic bug */
-	percpu_counter_add(&space_info->total_bytes_pinned, num_bytes);
-}
-
-
 static int __btrfs_free_extent(struct btrfs_trans_handle *trans,
 				struct btrfs_fs_info *info,
 				struct btrfs_delayed_ref_node *node, u64 parent,

Signature · 1 line

2.13.0

From: Omar Sandoval <[email protected]>

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/extent-tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

fs/btrfs/extent-tree.c +0 -0

index 7c01b4e9e3b6..6032e9a635f2 100644

From: Omar Sandoval <[email protected]>

Signed-off-by Omar Sandoval <[email protected]>

From: Omar Sandoval <[email protected]>

The extents marked in pin_down_extent() will be unpinned later in unpin_extent_range(), which decrements total_bytes_pinned. pin_down_extent() must increment the counter to avoid underflowing it. Also adjust btrfs_free_tree_block() to avoid accounting for the same extent twice.

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/extent-tree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

fs/btrfs/extent-tree.c +2 -1

index 6032e9a635f2..9cff937415cb 100644
@@ -6343,6 +6343,7 @@ static int pin_down_extent(struct btrfs_fs_info *fs_info,
 
 	trace_btrfs_space_reservation(fs_info, "pinned",
 				      cache->space_info->flags, num_bytes, 1);
+	percpu_counter_add(&cache->space_info->total_bytes_pinned, num_bytes);
 	set_extent_dirty(fs_info->pinned_extents, bytenr,
 			 bytenr + num_bytes - 1, GFP_NOFS | __GFP_NOFAIL);
 	return 0;

@@ -7189,6 +7190,7 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
 				goto out;
 		}
 
+		pin = 0;
 		cache = btrfs_lookup_block_group(fs_info, buf->start);
 
 		if (btrfs_header_flag(buf, BTRFS_HEADER_FLAG_WRITTEN)) {

@@ -7204,7 +7206,6 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
 		btrfs_free_reserved_bytes(cache, buf->len, 0);
 		btrfs_put_block_group(cache);
 		trace_btrfs_reserved_extent_free(fs_info, buf->start, buf->len);
-		pin = 0;
 	}
 out:
 	if (pin)

Signature · 1 line

2.13.0

From: Omar Sandoval <[email protected]>

Currently, we only increment total_bytes_pinned in btrfs_free_tree_block() when dropping the last reference on the block. However, when the delayed ref is run later, we will decrement total_bytes_pinned regardless of whether it was the last reference or not. This causes the counter to underflow when the reference we dropped was not the last reference. Fix it by incrementing the counter unconditionally, which is what btrfs_free_extent() does. This makes total_bytes_pinned an overestimate when references to shared extents are dropped, but in the worst case this will just make us try to commit the transaction to try to free up space and find we didn't free enough.

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/extent-tree.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

fs/btrfs/extent-tree.c +8 -9

index 9cff937415cb..52f3a0486e64 100644
@@ -7178,10 +7178,7 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
 		BUG_ON(ret); /* -ENOMEM */
 	}
 
-	if (!last_ref)
-		return;
-
-	if (btrfs_header_generation(buf) == trans->transid) {
+	if (last_ref && btrfs_header_generation(buf) == trans->transid) {
 		struct btrfs_block_group_cache *cache;
 
 		if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID) {

@@ -7212,11 +7209,13 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
 		add_pinned_bytes(fs_info, buf->len, btrfs_header_level(buf),
 				 root->root_key.objectid);
 
-	/*
-	 * Deleting the buffer, clear the corrupt flag since it doesn't matter
-	 * anymore.
-	 */
-	clear_bit(EXTENT_BUFFER_CORRUPT, &buf->bflags);
+	if (last_ref) {
+		/*
+		 * Deleting the buffer, clear the corrupt flag since it doesn't
+		 * matter anymore.
+		 */
+		clear_bit(EXTENT_BUFFER_CORRUPT, &buf->bflags);
+	}
 }
 
 /* Can return -ENOMEM */

Signature · 1 line

2.13.0

From: Omar Sandoval <[email protected]>

We need this to decide when to account pinned bytes.

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/delayed-ref.c | 29 ++++++++++++++++++++--------
 fs/btrfs/delayed-ref.h |  6 ++++--
 fs/btrfs/extent-tree.c | 51 ++++++++++++++++++++++++++------------------------
 3 files changed, 52 insertions(+), 34 deletions(-)

fs/btrfs/delayed-ref.c +10 -3

index be70d90dfee5..93ffa898df6d 100644
@@ -470,7 +470,8 @@ add_delayed_ref_tail_merge(struct btrfs_trans_handle *trans,
 static noinline void
 update_existing_head_ref(struct btrfs_delayed_ref_root *delayed_refs,
 			 struct btrfs_delayed_ref_node *existing,
-			 struct btrfs_delayed_ref_node *update)
+			 struct btrfs_delayed_ref_node *update,
+			 int *old_ref_mod_ret)
 {
 	struct btrfs_delayed_ref_head *existing_ref;
 	struct btrfs_delayed_ref_head *ref;

@@ -523,6 +524,8 @@ update_existing_head_ref(struct btrfs_delayed_ref_root *delayed_refs,
 	 * currently, for refs we just added we know we're a-ok.
 	 */
 	old_ref_mod = existing_ref->total_ref_mod;
+	if (old_ref_mod_ret)
+		*old_ref_mod_ret = old_ref_mod;
 	existing->ref_mod += update->ref_mod;
 	existing_ref->total_ref_mod += update->ref_mod;
 

@@ -550,7 +553,8 @@ add_delayed_ref_head(struct btrfs_fs_info *fs_info,
 		     struct btrfs_delayed_ref_node *ref,
 		     struct btrfs_qgroup_extent_record *qrecord,
 		     u64 bytenr, u64 num_bytes, u64 ref_root, u64 reserved,
-		     int action, int is_data, int *qrecord_inserted_ret)
+		     int action, int is_data, int *qrecord_inserted_ret,
+		     int *old_ref_mod, int *new_ref_mod)
 {
 	struct btrfs_delayed_ref_head *existing;
 	struct btrfs_delayed_ref_head *head_ref = NULL;

@@ -638,7 +642,8 @@ add_delayed_ref_head(struct btrfs_fs_info *fs_info,
 	if (existing) {
 		WARN_ON(ref_root && reserved && existing->qgroup_ref_root
 			&& existing->qgroup_reserved);
-		update_existing_head_ref(delayed_refs, &existing->node, ref);
+		update_existing_head_ref(delayed_refs, &existing->node, ref,
+					 old_ref_mod);
 		/*
 		 * we've updated the existing ref, free the newly
 		 * allocated ref

@@ -646,6 +651,8 @@ add_delayed_ref_head(struct btrfs_fs_info *fs_info,
 		kmem_cache_free(btrfs_delayed_ref_head_cachep, head_ref);
 		head_ref = existing;
 	} else {
+		if (old_ref_mod)
+			*old_ref_mod = 0;
 		if (is_data && count_mod < 0)
 			delayed_refs->pending_csums += num_bytes;
 		delayed_refs->num_heads++;

fs/btrfs/delayed-ref.c +9 -5

 	return head_ref;
 }
 

@@ -778,7 +787,8 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
 			       struct btrfs_trans_handle *trans,
 			       u64 bytenr, u64 num_bytes, u64 parent,
 			       u64 ref_root,  int level, int action,
-			       struct btrfs_delayed_extent_op *extent_op)
+			       struct btrfs_delayed_extent_op *extent_op,
+			       int *old_ref_mod, int *new_ref_mod)
 {
 	struct btrfs_delayed_tree_ref *ref;
 	struct btrfs_delayed_ref_head *head_ref;

@@ -813,7 +823,8 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
 	 */
 	head_ref = add_delayed_ref_head(fs_info, trans, &head_ref->node, record,
 					bytenr, num_bytes, 0, 0, action, 0,
-					&qrecord_inserted);
+					&qrecord_inserted, old_ref_mod,
+					new_ref_mod);
 
 	add_delayed_tree_ref(fs_info, trans, head_ref, &ref->node, bytenr,
 			     num_bytes, parent, ref_root, level, action);

@@ -838,7 +849,8 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
 			       struct btrfs_trans_handle *trans,
 			       u64 bytenr, u64 num_bytes,
 			       u64 parent, u64 ref_root,
-			       u64 owner, u64 offset, u64 reserved, int action)
+			       u64 owner, u64 offset, u64 reserved, int action,
+			       int *old_ref_mod, int *new_ref_mod)
 {
 	struct btrfs_delayed_data_ref *ref;
 	struct btrfs_delayed_ref_head *head_ref;

@@ -878,7 +890,8 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
 	 */
 	head_ref = add_delayed_ref_head(fs_info, trans, &head_ref->node, record,
 					bytenr, num_bytes, ref_root, reserved,
-					action, 1, &qrecord_inserted);
+					action, 1, &qrecord_inserted,
+					old_ref_mod, new_ref_mod);
 
 	add_delayed_data_ref(fs_info, trans, head_ref, &ref->node, bytenr,
 				   num_bytes, parent, ref_root, owner, offset,

@@ -909,7 +922,7 @@ int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
 
 	add_delayed_ref_head(fs_info, trans, &head_ref->node, NULL, bytenr,
 			     num_bytes, 0, 0, BTRFS_UPDATE_DELAYED_HEAD,
-			     extent_op->is_data, NULL);
+			     extent_op->is_data, NULL, NULL, NULL);
 
 	spin_unlock(&delayed_refs->lock);
 	return 0;

fs/btrfs/delayed-ref.h +4 -2

index c0264ff01b53..ce88e4ac5276 100644
@@ -247,12 +247,14 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
 			       struct btrfs_trans_handle *trans,
 			       u64 bytenr, u64 num_bytes, u64 parent,
 			       u64 ref_root, int level, int action,
-			       struct btrfs_delayed_extent_op *extent_op);
+			       struct btrfs_delayed_extent_op *extent_op,
+			       int *old_ref_mod, int *new_ref_mod);
 int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
 			       struct btrfs_trans_handle *trans,
 			       u64 bytenr, u64 num_bytes,
 			       u64 parent, u64 ref_root,
-			       u64 owner, u64 offset, u64 reserved, int action);
+			       u64 owner, u64 offset, u64 reserved, int action,
+			       int *old_ref_mod, int *new_ref_mod);
 int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
 				struct btrfs_trans_handle *trans,
 				u64 bytenr, u64 num_bytes,

fs/btrfs/extent-tree.c +27 -24

index 52f3a0486e64..6dce7abafe84 100644
@@ -2119,14 +2119,16 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
 
 	if (owner < BTRFS_FIRST_FREE_OBJECTID) {
 		ret = btrfs_add_delayed_tree_ref(fs_info, trans, bytenr,
-					num_bytes,
-					parent, root_objectid, (int)owner,
-					BTRFS_ADD_DELAYED_REF, NULL);
+						 num_bytes, parent,
+						 root_objectid, (int)owner,
+						 BTRFS_ADD_DELAYED_REF, NULL,
+						 NULL, NULL);
 	} else {
 		ret = btrfs_add_delayed_data_ref(fs_info, trans, bytenr,
-					num_bytes, parent, root_objectid,
-					owner, offset, 0,
-					BTRFS_ADD_DELAYED_REF);
+						 num_bytes, parent,
+						 root_objectid, owner, offset,
+						 0, BTRFS_ADD_DELAYED_REF, NULL,
+						 NULL);
 	}
 	return ret;
 }

@@ -7169,12 +7171,12 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
 	int ret;
 
 	if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID) {
-		ret = btrfs_add_delayed_tree_ref(fs_info, trans,
-						 buf->start, buf->len,
-						 parent,
+		ret = btrfs_add_delayed_tree_ref(fs_info, trans, buf->start,
+						 buf->len, parent,
 						 root->root_key.objectid,
 						 btrfs_header_level(buf),
-						 BTRFS_DROP_DELAYED_REF, NULL);
+						 BTRFS_DROP_DELAYED_REF, NULL,
+						 NULL, NULL);
 		BUG_ON(ret); /* -ENOMEM */
 	}
 

@@ -7242,15 +7244,16 @@ int btrfs_free_extent(struct btrfs_trans_handle *trans,
 		ret = 0;
 	} else if (owner < BTRFS_FIRST_FREE_OBJECTID) {
 		ret = btrfs_add_delayed_tree_ref(fs_info, trans, bytenr,
-					num_bytes,
-					parent, root_objectid, (int)owner,
-					BTRFS_DROP_DELAYED_REF, NULL);
+						 num_bytes, parent,
+						 root_objectid, (int)owner,
+						 BTRFS_DROP_DELAYED_REF, NULL,
+						 NULL, NULL);
 	} else {
 		ret = btrfs_add_delayed_data_ref(fs_info, trans, bytenr,
-						num_bytes,
-						parent, root_objectid, owner,
-						offset, 0,
-						BTRFS_DROP_DELAYED_REF);
+						 num_bytes, parent,
+						 root_objectid, owner, offset,
+						 0, BTRFS_DROP_DELAYED_REF,
+						 NULL, NULL);
 	}
 	return ret;
 }

@@ -8198,9 +8201,9 @@ int btrfs_alloc_reserved_file_extent(struct btrfs_trans_handle *trans,
 	BUG_ON(root_objectid == BTRFS_TREE_LOG_OBJECTID);
 
 	ret = btrfs_add_delayed_data_ref(fs_info, trans, ins->objectid,
-					 ins->offset, 0,
-					 root_objectid, owner, offset,
-					 ram_bytes, BTRFS_ADD_DELAYED_EXTENT);
+					 ins->offset, 0, root_objectid, owner,
+					 offset, ram_bytes,
+					 BTRFS_ADD_DELAYED_EXTENT, NULL, NULL);
 	return ret;
 }
 

@@ -8420,11 +8423,11 @@ struct extent_buffer *btrfs_alloc_tree_block(struct btrfs_trans_handle *trans,
 		extent_op->is_data = false;
 		extent_op->level = level;
 
-		ret = btrfs_add_delayed_tree_ref(fs_info, trans,
-						 ins.objectid, ins.offset,
-						 parent, root_objectid, level,
+		ret = btrfs_add_delayed_tree_ref(fs_info, trans, ins.objectid,
+						 ins.offset, parent,
+						 root_objectid, level,
 						 BTRFS_ADD_DELAYED_EXTENT,
-						 extent_op);
+						 extent_op, NULL, NULL);
 		if (ret)
 			goto out_free_delayed;
 	}

Signature · 1 line

2.13.0

From: Omar Sandoval <[email protected]>

The total_bytes_pinned counter is completely broken when accounting delayed refs:

- If two drops for the same extent are merged, we will decrement total_bytes_pinned twice but only increment it once.
- If an add is merged into a drop or vice versa, we will decrement the total_bytes_pinned counter but never increment it.
- If multiple references to an extent are dropped, we will account it multiple times, potentially vastly over-estimating the number of bytes that will be freed by a commit and doing unnecessary work when we're close to ENOSPC.

The last issue is relatively minor, but the first two make the total_bytes_pinned counter leak or underflow very often. These accounting issues were introduced in b150a4f10d87 ("Btrfs: use a percpu to keep track of possibly pinned bytes"), but they were papered over by zeroing out the counter on every commit until d288db5dc011 ("Btrfs: fix race of using total_bytes_pinned").

We need to make sure that an extent is accounted as pinned exactly once if and only if we will drop references to it when when the transaction is committed. Ideally we would only add to total_bytes_pinned when the *last* reference is dropped, but this information isn't readily available for data extents. Again, this over-estimation can lead to extra commits when we're close to ENOSPC, but it's not as bad as before.

The fix implemented here is to increment total_bytes_pinned when the total refmod count for an extent goes negative and decrement it if the refmod count goes back to non-negative or after we've run all of the delayed refs for that extent.

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/extent-tree.c | 41 ++++++++++++++++++++++++++++++++---------
 1 file changed, 32 insertions(+), 9 deletions(-)

fs/btrfs/extent-tree.c +32 -9

index 6dce7abafe84..75ad24f8d253 100644
@@ -2112,6 +2112,7 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
 			 u64 bytenr, u64 num_bytes, u64 parent,
 			 u64 root_objectid, u64 owner, u64 offset)
 {
+	int old_ref_mod, new_ref_mod;
 	int ret;
 
 	BUG_ON(owner < BTRFS_FIRST_FREE_OBJECTID &&

@@ -2122,14 +2123,18 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
 						 num_bytes, parent,
 						 root_objectid, (int)owner,
 						 BTRFS_ADD_DELAYED_REF, NULL,
-						 NULL, NULL);
+						 &old_ref_mod, &new_ref_mod);
 	} else {
 		ret = btrfs_add_delayed_data_ref(fs_info, trans, bytenr,
 						 num_bytes, parent,
 						 root_objectid, owner, offset,
-						 0, BTRFS_ADD_DELAYED_REF, NULL,
-						 NULL);
+						 0, BTRFS_ADD_DELAYED_REF,
+						 &old_ref_mod, &new_ref_mod);
 	}
+
+	if (ret == 0 && old_ref_mod < 0 && new_ref_mod >= 0)
+		add_pinned_bytes(fs_info, -num_bytes, owner, root_objectid);
+
 	return ret;
 }
 

@@ -2433,6 +2438,16 @@ static int run_one_delayed_ref(struct btrfs_trans_handle *trans,
 		head = btrfs_delayed_node_to_head(node);
 		trace_run_delayed_ref_head(fs_info, node, head, node->action);
 
+		if (head->total_ref_mod < 0) {
+			struct btrfs_block_group_cache *cache;
+
+			cache = btrfs_lookup_block_group(fs_info, node->bytenr);
+			ASSERT(cache);
+			percpu_counter_add(&cache->space_info->total_bytes_pinned,
+					   -node->num_bytes);
+			btrfs_put_block_group(cache);
+		}
+
 		if (insert_reserved) {
 			btrfs_pin_extent(fs_info, node->bytenr,
 					 node->num_bytes, 1);

@@ -6269,6 +6284,8 @@ static int update_block_group(struct btrfs_trans_handle *trans,
 			trace_btrfs_space_reservation(info, "pinned",
 						      cache->space_info->flags,
 						      num_bytes, 1);
+			percpu_counter_add(&cache->space_info->total_bytes_pinned,
+					   num_bytes);
 			set_extent_dirty(info->pinned_extents,
 					 bytenr, bytenr + num_bytes - 1,
 					 GFP_NOFS | __GFP_NOFAIL);

@@ -7038,8 +7055,6 @@ static int __btrfs_free_extent(struct btrfs_trans_handle *trans,
 				goto out;
 			}
 		}
-		add_pinned_bytes(info, -num_bytes, owner_objectid,
-				 root_objectid);
 	} else {
 		if (found_extent) {
 			BUG_ON(is_data && refs_to_drop !=

@@ -7171,13 +7186,16 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
 	int ret;
 
 	if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID) {
+		int old_ref_mod, new_ref_mod;
+
 		ret = btrfs_add_delayed_tree_ref(fs_info, trans, buf->start,
 						 buf->len, parent,
 						 root->root_key.objectid,
 						 btrfs_header_level(buf),
 						 BTRFS_DROP_DELAYED_REF, NULL,
-						 NULL, NULL);
+						 &old_ref_mod, &new_ref_mod);
 		BUG_ON(ret); /* -ENOMEM */
+		pin = old_ref_mod >= 0 && new_ref_mod < 0;
 	}
 
 	if (last_ref && btrfs_header_generation(buf) == trans->transid) {

@@ -7226,12 +7244,12 @@ int btrfs_free_extent(struct btrfs_trans_handle *trans,
 		      u64 bytenr, u64 num_bytes, u64 parent, u64 root_objectid,
 		      u64 owner, u64 offset)
 {
+	int old_ref_mod, new_ref_mod;
 	int ret;
 
 	if (btrfs_is_testing(fs_info))
 		return 0;
 
-	add_pinned_bytes(fs_info, num_bytes, owner, root_objectid);
 
 	/*
 	 * tree log blocks never actually go into the extent allocation

@@ -7241,20 +7259,25 @@ int btrfs_free_extent(struct btrfs_trans_handle *trans,
 		WARN_ON(owner >= BTRFS_FIRST_FREE_OBJECTID);
 		/* unlocks the pinned mutex */
 		btrfs_pin_extent(fs_info, bytenr, num_bytes, 1);
+		old_ref_mod = new_ref_mod = 0;
 		ret = 0;
 	} else if (owner < BTRFS_FIRST_FREE_OBJECTID) {
 		ret = btrfs_add_delayed_tree_ref(fs_info, trans, bytenr,
 						 num_bytes, parent,
 						 root_objectid, (int)owner,
 						 BTRFS_DROP_DELAYED_REF, NULL,
-						 NULL, NULL);
+						 &old_ref_mod, &new_ref_mod);
 	} else {
 		ret = btrfs_add_delayed_data_ref(fs_info, trans, bytenr,
 						 num_bytes, parent,
 						 root_objectid, owner, offset,
 						 0, BTRFS_DROP_DELAYED_REF,
-						 NULL, NULL);
+						 &old_ref_mod, &new_ref_mod);
 	}
+
+	if (ret == 0 && old_ref_mod >= 0 && new_ref_mod < 0)
+		add_pinned_bytes(fs_info, num_bytes, owner, root_objectid);
+
 	return ret;
 }
 

Signature · 1 line

2.13.0

Quoted 28 lines · Omar Sandoval

From: Omar Sandoval <[email protected]>

The total_bytes_pinned counter is completely broken when accounting delayed refs:

- If two drops for the same extent are merged, we will decrement total_bytes_pinned twice but only increment it once.
- If an add is merged into a drop or vice versa, we will decrement the total_bytes_pinned counter but never increment it.
- If multiple references to an extent are dropped, we will account it multiple times, potentially vastly over-estimating the number of bytes that will be freed by a commit and doing unnecessary work when we're close to ENOSPC.

The last issue is relatively minor, but the first two make the total_bytes_pinned counter leak or underflow very often. These accounting issues were introduced in b150a4f10d87 ("Btrfs: use a percpu to keep track of possibly pinned bytes"), but they were papered over by zeroing out the counter on every commit until d288db5dc011 ("Btrfs: fix race of using total_bytes_pinned").

We need to make sure that an extent is accounted as pinned exactly once if and only if we will drop references to it when when the transaction is committed. Ideally we would only add to total_bytes_pinned when the *last* reference is dropped, but this information isn't readily available for data extents. Again, this over-estimation can lead to extra commits when we're close to ENOSPC, but it's not as bad as before.

Quoted 5 lines

The fix implemented here is to increment total_bytes_pinned when the total refmod count for an extent goes negative and decrement it if the refmod count goes back to non-negative or after we've run all of the delayed refs for that extent.

The patch could be cleaner if we inc/dec %pinned inside delayed_ref.c.

The idea looks good to me.

Reviewed-by Liu Bo <[email protected]>

From: Omar Sandoval <[email protected]>

Catch any future/remaining leaks or underflows of total_bytes_pinned.

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/extent-tree.c | 1 +
 1 file changed, 1 insertion(+)

fs/btrfs/extent-tree.c +1 -0

index 75ad24f8d253..5fb2fb27eda6 100644
@@ -9860,6 +9860,7 @@ int btrfs_free_block_groups(struct btrfs_fs_info *info)
 			    space_info->bytes_reserved > 0 ||
 			    space_info->bytes_may_use > 0))
 			dump_space_info(info, space_info, 0, 0);
+		WARN_ON(percpu_counter_sum(&space_info->total_bytes_pinned) != 0);
 		list_del(&space_info->list);
 		for (i = 0; i < BTRFS_NR_RAID_TYPES; i++) {
 			struct kobject *kobj;

Signature · 1 line

2.13.0

Quoted 4 lines · Omar Sandoval

From: Omar Sandoval <[email protected]>

Catch any future/remaining leaks or underflows of total_bytes_pinned.

This might be a little bit late, what about checking it after btrfs_finish_extetn_commit()?

-liubo

From: Omar Sandoval <[email protected]>

Catch any future/remaining leaks or underflows of total_bytes_pinned.

Signed-off-by Omar Sandoval <[email protected]>

---
 fs/btrfs/extent-tree.c | 1 +
 1 file changed, 1 insertion(+)

fs/btrfs/extent-tree.c +0 -0

index 75ad24f8d253..5fb2fb27eda6 100644

From: Omar Sandoval <[email protected]>

Catch any future/remaining leaks or underflows of total_bytes_pinned.

Signed-off-by Omar Sandoval <[email protected]>

From: Omar Sandoval <[email protected]>

This series fixes several problems with the total_bytes_pinned counter. Patches 1 and 2 are cleanups. Patches 3 and 4 are straightforward fixes. Patch 5 is prep for patch 6. Patch 6 is the most complicated fix. Patches 5 and 6 are ugly, I'd love any suggestions for a cleaner fix. Finally, patch 7 adds a warning to catch similar issues in the future.

Quoted 7 lines · Omar Sandoval

From: Omar Sandoval <[email protected]>

This series fixes several problems with the total_bytes_pinned counter. Patches 1 and 2 are cleanups. Patches 3 and 4 are straightforward fixes. Patch 5 is prep for patch 6. Patch 6 is the most complicated fix. Patches 5 and 6 are ugly, I'd love any suggestions for a cleaner fix. Finally, patch 7 adds a warning to catch similar issues in the future.

Since this didn't really change the code significantly from the previous version (except for the addition of minor patches 2+7) have a

Tested-by Holger Hoffstätte <[email protected]>