On 7.05.19 г. 20:27 ч., Josef Bacik wrote:
> We have been seeing issues in production where a cleaner script will end
> up unlinking a bunch of files that have pending iputs. This means they
> will get their final iput's run at btrfs-cleaner time and thus are not
> throttled, which impacts the workload.
>
> Since we are unlinking these files we can just drop the delayed iput at
> unlink time. We are already holding a reference to the inode so this
> will not be the final iput and thus is completely safe to do at this
> point. Doing this means we are more likely to be doing the final iput
> at unlink time, and thus will get the IO charged to the caller and get
> throttled appropriately without affecting the main workload.
>
> Signed-off-by: Josef Bacik <jo...@toxicpanda.com>
> ---
> fs/btrfs/inode.c | 22 ++++++++++++++++++++++
> 1 file changed, 22 insertions(+)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index b6d549c993f6..e58685b5d398 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -4009,6 +4009,28 @@ static int __btrfs_unlink_inode(struct
> btrfs_trans_handle *trans,
> ret = 0;
> else if (ret)
> btrfs_abort_transaction(trans, ret);
> +
> + /*
> + * If we have a pending delayed iput we could end up with the final iput
> + * being run in btrfs-cleaner context. If we have enough of these built
> + * up we can end up burning a lot of time in btrfs-cleaner without any
> + * way to throttle the unlinks. Since we're currently holding a ref on
> + * the inode we can run the delayed iput here without any issues as the
> + * final iput won't be done until after we drop the ref we're currently
> + * holding.
> + */
FWIW the caller is not really holding an explicit reference, rather
there is a reference held by the dentry which is going to be disposed of
by vfs. Considering this I'd say this is a false claim. I.e "we" do not
hold a reference.
> + if (!list_empty(&inode->delayed_iput)) {
> + spin_lock(&fs_info->delayed_iput_lock);
> + if (!list_empty(&inode->delayed_iput)) {
> + list_del_init(&inode->delayed_iput);
> + spin_unlock(&fs_info->delayed_iput_lock);
> + iput(&inode->vfs_inode);
> + if (atomic_dec_and_test(&fs_info->nr_delayed_iputs))
> + wake_up(&fs_info->delayed_iputs_wait);
> + } else {
> + spin_unlock(&fs_info->delayed_iput_lock);
> + }
> + }
OTOH this really feels like a hack and this stems from the fact that
iput is rather rudimentary. Additionally you are essentially opencoding
the body of btrfs_run_delayed_iputs. I was going to suggest to introduce
a new helper factoring out the common code but it will get ugly due to
the spin lock being dropped before doing the iput.
But then I'm really starting to question the utility of delayed iputs.
Presumably it was added to defer the expensive final iput in the cleaner
context or avoid some deadlocks (but we don't know which exactly). Yet,
here we are some time later where you are essentially saying "this
mechanism is suboptimal because it's dumb and instead of improving
things it's making them worse in certain cases, so let's unload it a bit
by doing an iput here".
> err:
> btrfs_free_path(path);
> if (ret)
>
