btrfs: don't force mounts to wait for cleaner_kthread to delete one or more subvolumes

On Wed, Jun 1, 2016 at 5:39 AM, Zygo Blaxell
<ce3g8jdj@umail.furryterror.org> wrote:
> On Thu, May 05, 2016 at 12:23:49AM -0400, Zygo Blaxell wrote:
>> During a mount, we start the cleaner kthread first because the transaction
>> kthread wants to wake up the cleaner kthread.  We start the transaction
>> kthread next because everything in btrfs wants transactions.  We do reloc
>> recovery in the thread that was doing the original mount call once the
>> transaction kthread is running.  This means that the cleaner kthread
>> could already be running when reloc recovery happens (e.g. if a snapshot
>> delete was started before a crash).
>>
>> Relocation does not play well with the cleaner kthread, so a mutex was
>> added in commit 5f3164813b90f7dbcb5c3ab9006906222ce471b7 "Btrfs: fix
>> race between balance recovery and root deletion" to prevent both from
>> being active at the same time.
>>
>> If the cleaner kthread is already holding the mutex by the time we get
>> to btrfs_recover_relocation, the mount will be blocked until at least
>> one deleted subvolume is cleaned (possibly more if the mount process
>> doesn't get the lock right away).  During this time (which could be an
>> arbitrarily long time on a large/slow filesystem), the mount process is
>> stuck and the filesystem is unnecessarily inaccessible.
>>
>> Fix this by locking cleaner_mutex before we start cleaner_kthread, and
>> unlocking the mutex after mount no longer requires it.  This ensures
>> that the mounting process will not be blocked by the cleaner kthread.
>> The cleaner kthread is already prepared for mutex contention and will
>> just go to sleep until the mutex is available.
>> ---
>>  fs/btrfs/disk-io.c | 18 +++++++++++++++---
>>  1 file changed, 15 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index d8d68af..7c8f435 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -2509,6 +2509,7 @@ int open_ctree(struct super_block *sb,
>>       int num_backups_tried = 0;
>>       int backup_index = 0;
>>       int max_active;
>> +     bool cleaner_mutex_locked = false;
>>
>>       tree_root = fs_info->tree_root = btrfs_alloc_root(fs_info);
>>       chunk_root = fs_info->chunk_root = btrfs_alloc_root(fs_info);
>> @@ -2988,6 +2989,13 @@ retry_root_backup:
>>               goto fail_sysfs;
>>       }
>>
>> +     /*
>> +      * Hold the cleaner_mutex thread here so that we don't block
>> +      * for a long time on btrfs_recover_relocation.  cleaner_kthread
>> +      * will wait for us to finish mounting the filesystem.
>> +      */
>> +     mutex_lock(&fs_info->cleaner_mutex);
>> +     cleaner_mutex_locked = true;
>>       fs_info->cleaner_kthread = kthread_run(cleaner_kthread, tree_root,
>>                                              "btrfs-cleaner");
>>       if (IS_ERR(fs_info->cleaner_kthread))
>
> Unfortunately, if we have a log to replay, we get to code like this
> in open_ctree:
>
>         /* do not make disk changes in broken FS */
>         if (btrfs_super_log_root(disk_super) != 0) {
>                 ret = btrfs_replay_log(fs_info, fs_devices);
>                 if (ret) {
>                         err = ret;
>                         goto fail_qgroup;
>                 }
>         }
>
> and:
>
>         static int btrfs_replay_log(struct btrfs_fs_info *fs_info,
>                                     struct btrfs_fs_devices *fs_devices)
>         {
>         [...]
>                 if (fs_info->sb->s_flags & MS_RDONLY) {
>                         ret = btrfs_commit_super(tree_root);
>                         if (ret)
>                                 return ret;
>                 }
>
> and finally:
>
>         int btrfs_commit_super(struct btrfs_root *root)
>         {
>                 struct btrfs_trans_handle *trans;
>
>                 mutex_lock(&root->fs_info->cleaner_mutex);
>                 btrfs_run_delayed_iputs(root);
>                 mutex_unlock(&root->fs_info->cleaner_mutex);
>                 wake_up_process(root->fs_info->cleaner_kthread);
>
> Well, dammit.  Since we have already locked cleaner_mutex, it promptly
> recursive-deadlocks on itself--but only if the filesystem was not cleanly
> umounted, and the problem disappears if you reboot and try to mount again
> because there won't be a log to replay the second time.
>
> Could we just add a bool to fs_info that says to cleaner_kthread "don't
> do anything yet, we're not finished mounting"?  That way it doesn't break
> if some new place to lock cleaner_mutex pops up (they do seem to move
> around from one kernel version to the next).
>
> I think we can do btrfs_run_delayed_iputs and just skip the
> wake_up_process call here?  Or neuter it by having cleaner_kthread do
> nothing while we are still somewhere in the middle of open_ctree.

You can try something as simple as (untested):

>

btrfs: don't force mounts to wait for cleaner_kthread to delete one or more subvolumes

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