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

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Date:   Sun, 13 Nov 2016 11:11:37 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Konstantin Khlebnikov <koct9i@...il.com>,
        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 2:42 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Fri, Nov 11, 2016 at 12:06 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
>> On Fri, Nov 11, 2016 at 10:46 AM, Konstantin Khlebnikov
>> <koct9i@...il.com> wrote:
>>> 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.
>>
>> We do need something to prevent multiple copy-ups starting up in
>> parallel on the same file, though.
>>
>
> I guess an inode_lock on the copy-up victim should suffice?

Just to follow up on this hijacked thread.
I posted patches to lock the overlay inode of copied up file
and relaxed the lock_rename during data copy up.

Amir.