Re: [PATCH] btrfs: resume qgroup rescan on rw remount

[Nikolay Borisov]

On 10.07.2017 16:12, Nikolay Borisov wrote:
> 
> 
> On  4.07.2017 14:49, Aleksa Sarai wrote:
>> Several distributions mount the "proper root" as ro during initrd and
>> then remount it as rw before pivot_root(2). Thus, if a rescan had been
>> aborted by a previous shutdown, the rescan would never be resumed.
>>
>> This issue would manifest itself as several btrfs ioctl(2)s causing the
>> entire machine to hang when btrfs_qgroup_wait_for_completion was hit
>> (due to the fs_info->qgroup_rescan_running flag being set but the rescan
>> itself not being resumed). Notably, Docker's btrfs storage driver makes
>> regular use of BTRFS_QUOTA_CTL_DISABLE and BTRFS_IOC_QUOTA_RESCAN_WAIT
>> (causing this problem to be manifested on boot for some machines).
>>
>> Cc: <stable@xxxxxxxxxxxxxxx> # v3.11+
>> Cc: Jeff Mahoney <jeffm@xxxxxxxx>
>> Fixes: b382a324b60f ("Btrfs: fix qgroup rescan resume on mount")
>> Signed-off-by: Aleksa Sarai <asarai@xxxxxxx>
> 
> Indeed, looking at the code it seems that b382a324b60f ("Btrfs: fix
> qgroup rescan resume on mount") missed adding the qgroup_rescan_resume
> in the remount path. One thing which I couldn't verify though is whether
> reading fs_info->qgroup_flags without any locking is safe from remount
> context.
> 
> During remount I don't see any locks taken that prevent operations which
> can modify qgroup_flags.
> 
> 

Further inspection reveals that the access rules to qgroup_flags are
somewhat broken so this patch doesn't really make things any worse than
they are. As such:

Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
Tested-by: Nikolay Borisov <nborisov@xxxxxxxx>

> 
>> ---
>>  fs/btrfs/super.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
>> index 6346876c97ea..ff6690389343 100644
>> --- a/fs/btrfs/super.c
>> +++ b/fs/btrfs/super.c
>> @@ -1821,6 +1821,8 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
>>  			goto restore;
>>  		}
>>  
>> +		btrfs_qgroup_rescan_resume(fs_info);
>> +
>>  		if (!fs_info->uuid_root) {
>>  			btrfs_info(fs_info, "creating UUID tree");
>>  			ret = btrfs_create_uuid_tree(fs_info);
>>
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