Re: [PATCH 3/3] ovl: redirect on rename-dir

[Miklos Szeredi]

On Fri, Nov 11, 2016 at 10:46 AM, Konstantin Khlebnikov
<koct9i@xxxxxxxxx> wrote:
> On Fri, Nov 11, 2016 at 1:56 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>> On Mon, Nov 7, 2016 at 3:38 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>>> On Mon, Nov 7, 2016 at 12:08 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
>>>> On Mon, Nov 7, 2016 at 1:04 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>>>> On Mon, Nov 7, 2016 at 10:58 AM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
>>>>>
>>>>>> I've stumbled on somehow related problem - concurrent copy-ups are
>>>>>> strictly serialized by rename locks.
>>>>>> Obviously, file copying could be done in parallel: locks are required
>>>>>> only for final rename.
>>>>>> Because of that overlay slower that aufs for some workloads.
>>>>>
>>>>> Easy to fix: for each copy up create a separate subdir of "work".
>>>>> Then the contention is only for the time of creating the subdir, which
>>>>> is very short.
>>>>
>>>> Yeah, but lock_rename() also takes per-sb s_vfs_rename_mutex (kludge by Al Viro)
>>>> I think proper synchronization for concurrent copy-up (for example
>>>> round flag on ovl_entry) and  locking rename only for rename could be
>>>> better.
>>>
>>> Removing s_vfs_rename_mutex from copy-up path is something I have been
>>> pondering about.
>>> Assuming that I understand Al's comment above vfs_rename() correctly,
>>> the sole purpose of per-sb serialization is to prevent loop creations.
>>> However, how can one create a loop by moving a non-directory?
>>> So it looks like at least for the non-dir copy up case, a much finer grained
>>> lock is in order.
>>>
>>
>>
>> I posted patches to relax the s_vfs_rename_mutex for copy-up and
>> whiteout in some use cases.
>>
>> Konstantin,
>>
>> It would be useful to know if those patches help with your use case.
>>
>
> Well.. I think relaxing only s_vfs_rename_mutex wouldn't help much here.
> Copying is still serialized by i_mutex on workdir?
> Data copying should be done without rename locks at all.

We do need something to prevent multiple copy-ups starting up in
parallel on the same file, though.

Thanks,
Miklos