Btrfs: improve performance on fsync of files with multiple hardlinks

Commit 41bd6067692382 ("Btrfs: fix fsync of files with multiple hard links
in new directories") introduced a path that makes fsync fallback to a full
transaction commit in order to avoid losing hard links and new ancestors
of the fsynced inode. That path is triggered only when the inode has more
than one hard link and either has a new hard link created in the current
transaction or the inode was evicted and reloaded in the current
transaction.

That path ends up getting triggered very often (hundreds of times) during
the course of pgbench benchmarks, resulting in performance drops of about
20%.

This change restores the performance by not triggering the full transaction
commit in those cases, and instead iterate the fs/subvolume tree in search
of all possible new ancestors, for all hard links, to log them.

Reported-by: Zhao Yuhu <zyuhu@suse.com>
Tested-by: James Wang <jnwang@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 0 insertions, 17 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index fb237ffbda1a..fcc23e14a86c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3689,21 +3689,6 @@ cache_index:
 	 * inode is not a directory, logging its parent unnecessarily.
 	 */
 	BTRFS_I(inode)->last_unlink_trans = BTRFS_I(inode)->last_trans;
-	/*
-	 * Similar reasoning for last_link_trans, needs to be set otherwise
-	 * for a case like the following:
-	 *
-	 * mkdir A
-	 * touch foo
-	 * ln foo A/bar
-	 * echo 2 > /proc/sys/vm/drop_caches
-	 * fsync foo
-	 * <power failure>
-	 *
-	 * Would result in link bar and directory A not existing after the power
-	 * failure.
-	 */
-	BTRFS_I(inode)->last_link_trans = BTRFS_I(inode)->last_trans;
 
 	path->slots[0]++;
 	if (inode->i_nlink != 1 ||
@@ -6655,7 +6640,6 @@ static int btrfs_link(struct dentry *old_dentry, struct inode *dir,
 			if (err)
 				goto fail;
 		}
-		BTRFS_I(inode)->last_link_trans = trans->transid;
 		d_instantiate(dentry, inode);
 		ret = btrfs_log_new_name(trans, BTRFS_I(inode), NULL, parent,
 					 true, NULL);
@@ -9192,7 +9176,6 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
 	ei->index_cnt = (u64)-1;
 	ei->dir_index = 0;
 	ei->last_unlink_trans = 0;
-	ei->last_link_trans = 0;
 	ei->last_log_commit = 0;
 
 	spin_lock_init(&ei->lock);