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

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Date:   Fri, 11 Nov 2016 12:46:28 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Raphael Hertzog <hertzog@...ian.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        Guillem Jover <guillem@...ian.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] ovl: redirect on rename-dir

On Fri, Nov 11, 2016 at 1:56 AM, Amir Goldstein <amir73il@...il.com> wrote:
> On Mon, Nov 7, 2016 at 3:38 PM, Amir Goldstein <amir73il@...il.com> wrote:
>> On Mon, Nov 7, 2016 at 12:08 PM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
>>> On Mon, Nov 7, 2016 at 1:04 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
>>>> On Mon, Nov 7, 2016 at 10:58 AM, Konstantin Khlebnikov <koct9i@...il.com> 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.