[PATCH AUTOSEL 5.4 102/130] btrfs: fix race that makes inode logging fallback to transaction commit

Sasha Levin
2020-12-23T02:54:26Z

From: Filipe Manana <fdmanana@suse.com>

[ Upstream commit 47d3db41e190ca4a9c6e4a848052f4c5ca633db1 ]

When logging an inode and the previous transaction is still committing, we
have a time window where we can end up incorrectly think an inode has its
last_unlink_trans field with a value greater than the last transaction
committed, which results in the logging to fallback to a full transaction
commit, which is usually much more expensive than doing a log commit.

The race is described by the following steps:

1) We are at transaction 1000;

2) We modify an inode X (a directory) using transaction 1000 and set its
last_unlink_trans field to 1000, because for example we removed one
of its subdirectories;

3) We create a new inode Y with a dentry in inode X using transaction 1000,
so its generation field is set to 1000;

4) The commit for transaction 1000 is started by task A;

5) The task committing transaction 1000 sets the transaction state to
unblocked, writes the dirty extent buffers and the super blocks, then
unlocks tree_log_mutex;

6) Some task starts a new transaction with a generation of 1001;

7) We do some modification to inode Y (using transaction 1001);

8) The transaction 1000 commit starts unpinning extents. At this point
fs_info->last_trans_committed still has a value of 999;

9) Task B starts an fsync on inode Y, and gets a handle for transaction
1001. When it gets to check_parent_dirs_for_sync() it does the checking
of the ancestor dentries because the following check does not evaluate
to true:

if (S_ISREG(inode->vfs_inode.i_mode) &&
inode->generation <= last_committed &&
inode->last_unlink_trans <= last_committed)
goto out;

The generation value for inode Y is 1000 and last_committed, which has
the value read from fs_info->last_trans_committed, has a value of 999,
so that check evaluates to false and we proceed to check the ancestor
inodes.

Once we get to the first ancestor, inode X, we call
btrfs_must_commit_transaction() on it, which evaluates to true:

static bool btrfs_must_commit_transaction(...)
{
struct btrfs_fs_info *fs_info = inode->root->fs_info;
bool ret = false;

mutex_lock(&inode->log_mutex);
if (inode->last_unlink_trans > fs_info->last_trans_committed) {
/*
* Make sure any commits to the log are forced to be full
* commits.
*/
btrfs_set_log_full_commit(trans);
ret = true;
}
(...)

because inode's X last_unlink_trans has a value of 1000 and
fs_info->last_trans_committed still has a value of 999, it returns
true to check_parent_dirs_for_sync(), making it return 1 which is
propagated up to btrfs_sync_file(), causing it to fallback to a full
transaction commit of transaction 1001.

We should have not fallen back to commit transaction 1001, since inode
X had last_unlink_trans set to 1000 and the super blocks for
transaction 1000 were already written. So while not resulting in a
functional problem, it leads to a lot more work and higher latencies
for a fsync since committing a transaction is usually more expensive
than committing a log (if other filesystem changes happened under that
transaction).

Similar problem happens when logging directories, for the same reason as
btrfs_must_commit_transaction() returns true on an inode with its
last_unlink_trans having the generation of the previous transaction and
that transaction is still committing, unpinning its freed extents.

So fix this by comparing last_unlink_trans with the id of the current
transaction instead of fs_info->last_trans_committed.

This case is often hit when running dbench for a long enough duration, as
it does lots of rename and rmdir operations (both update the field
last_unlink_trans of an inode) and fsyncs of files and directories.

This patch belongs to a patch set that is comprised of the following
patches:

btrfs: fix race causing unnecessary inode logging during link and rename
btrfs: fix race that results in logging old extents during a fast fsync
btrfs: fix race that causes unnecessary logging of ancestor inodes
btrfs: fix race that makes inode logging fallback to transaction commit
btrfs: fix race leading to unnecessary transaction commit when logging inode
btrfs: do not block inode logging for so long during transaction commit

Performance results are mentioned in the change log of the last patch.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/tree-log.c | 20 ++++++++------------
1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 72e0ff38646a7..54095753f84f0 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -5418,11 +5418,10 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
static bool btrfs_must_commit_transaction(struct btrfs_trans_handle *trans,
struct btrfs_inode *inode)
{
- struct btrfs_fs_info *fs_info = inode->root->fs_info;
bool ret = false;

mutex_lock(&inode->log_mutex);
- if (inode->last_unlink_trans > fs_info->last_trans_committed) {
+ if (inode->last_unlink_trans >= trans->transid) {
/*
* Make sure any commits to the log are forced to be full
* commits.
@@ -5444,8 +5443,7 @@ static bool btrfs_must_commit_transaction(struct btrfs_trans_handle *trans,
static noinline int check_parent_dirs_for_sync(struct btrfs_trans_handle *trans,
struct btrfs_inode *inode,
struct dentry *parent,
- struct super_block *sb,
- u64 last_committed)
+ struct super_block *sb)
{
int ret = 0;
struct dentry *old_parent = NULL;
@@ -5457,8 +5455,8 @@ static noinline int check_parent_dirs_for_sync(struct btrfs_trans_handle *trans,
* and other fun in this file.
*/
if (S_ISREG(inode->vfs_inode.i_mode) &&
- inode->generation <= last_committed &&
- inode->last_unlink_trans <= last_committed)
+ inode->generation < trans->transid &&
+ inode->last_unlink_trans < trans->transid)
goto out;

if (!S_ISDIR(inode->vfs_inode.i_mode)) {
@@ -5993,7 +5991,6 @@ static int btrfs_log_inode_parent(struct btrfs_trans_handle *trans,
struct btrfs_fs_info *fs_info = root->fs_info;
struct super_block *sb;
int ret = 0;
- u64 last_committed = fs_info->last_trans_committed;
bool log_dentries = false;

sb = inode->vfs_inode.i_sb;
@@ -6018,8 +6015,7 @@ static int btrfs_log_inode_parent(struct btrfs_trans_handle *trans,
goto end_no_trans;
}

- ret = check_parent_dirs_for_sync(trans, inode, parent, sb,
- last_committed);
+ ret = check_parent_dirs_for_sync(trans, inode, parent, sb);
if (ret)
goto end_no_trans;

@@ -6049,8 +6045,8 @@ static int btrfs_log_inode_parent(struct btrfs_trans_handle *trans,
* and other fun in this file.
*/
if (S_ISREG(inode->vfs_inode.i_mode) &&
- inode->generation <= last_committed &&
- inode->last_unlink_trans <= last_committed) {
+ inode->generation < trans->transid &&
+ inode->last_unlink_trans < trans->transid) {
ret = 0;
goto end_trans;
}
@@ -6099,7 +6095,7 @@ static int btrfs_log_inode_parent(struct btrfs_trans_handle *trans,
* but the file inode does not have a matching BTRFS_INODE_REF_KEY item
* and has a link count of 2.
*/
- if (inode->last_unlink_trans > last_committed) {
+ if (inode->last_unlink_trans >= trans->transid) {
ret = btrfs_log_all_parents(trans, inode, ctx);
if (ret)
goto end_trans;
--
2.27.0