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

From: Amir Goldstein
Date: Mon Nov 07 2016 - 12:41:12 EST

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.

Anyway, it's on my todo list, as concurrent operation performance on overlayfs
is important to out use case.