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

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Date:   Fri, 11 Nov 2016 00:56:00 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Konstantin Khlebnikov <koct9i@...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 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.

Thanks,
Amir.