btrfs: remove racy and unnecessary inode transaction update when using no-holes

When using the NO_HOLES feature and expanding the size of an inode, we
update the inode's last_trans, last_sub_trans and last_log_commit fields
at maybe_insert_hole() so that a fsync does know that the inode needs to
be logged (by making sure that btrfs_inode_in_log() returns false). This
happens for expanding truncate operations, buffered writes, direct IO
writes and when cloning extents to an offset greater than the inode's
i_size.

However the way we do it is racy, because in between setting the inode's
last_sub_trans and last_log_commit fields, the log transaction ID that was
assigned to last_sub_trans might be committed before we read the root's
last_log_commit and assign that value to last_log_commit. If that happens
it would make a future call to btrfs_inode_in_log() return true. This is
a race that should be extremely unlikely to be hit in practice, and it is
the same that was described by commit bc0939fc ("btrfs: fix race
between marking inode needs to be logged and log syncing").

The fix would simply be to set last_log_commit to the value we assigned
to last_sub_trans minus 1, like it was done in that commit. However
updating these two fields plus the last_trans field is pointless here
because all the callers of btrfs_cont_expand() (which is the only
caller of maybe_insert_hole()) always call btrfs_set_inode_last_trans()
or btrfs_update_inode() after calling btrfs_cont_expand(). Calling either
btrfs_set_inode_last_trans() or btrfs_update_inode() guarantees that the
next fsync will log the inode, as it makes btrfs_inode_in_log() return
false.

So just remove the code that explicitly sets the inode's last_trans,
last_sub_trans and last_log_commit fields.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -5087,15 +5087,13 @@ static int maybe_insert_hole(struct btrfs_root *root, struct btrfs_inode *inode,
 	int ret;
 
 	/*
-	 * Still need to make sure the inode looks like it's been updated so
-	 * that any holes get logged if we fsync.
+	 * If NO_HOLES is enabled, we don't need to do anything.
+	 * Later, up in the call chain, either btrfs_set_inode_last_sub_trans()
+	 * or btrfs_update_inode() will be called, which guarantee that the next
+	 * fsync will know this inode was changed and needs to be logged.
 	 */
-	if (btrfs_fs_incompat(fs_info, NO_HOLES)) {
-		inode->last_trans = fs_info->generation;
-		inode->last_sub_trans = root->log_transid;
-		inode->last_log_commit = root->last_log_commit;
+	if (btrfs_fs_incompat(fs_info, NO_HOLES))
 		return 0;
-	}
 
 	/*
 	 * 1 - for the one we're dropping