Re: [RFC PATCH 3/3] overlay: Add the ability to remount volatile directories when safe

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Nov 16, 2020 at 09:42:40AM -0500, Vivek Goyal wrote:
> On Sun, Nov 15, 2020 at 08:57:58PM -0800, Sargun Dhillon wrote:
> > Overlayfs added the ability to setup mounts where all syncs could be
> > short-circuted in (2a99ddacee43: ovl: provide a mount option "volatile").
> > 
> > A user might want to remount this fs, but we do not let the user because
> > of the "incompat" detection feature. In the case of volatile, it is safe
> > to do something like[1]:
> > 
> > $ sync -f /root/upperdir
> > $ rm -rf /root/workdir/incompat/volatile
> > 
> > There are two ways to go about this. You can call sync on the underlying
> > filesystem, check the error code, and delete the dirty file if everything
> > is clean. If you're running lots of containers on the same filesystem, or
> > you want to avoid all unnecessary I/O, this may be suboptimal.
> > 
> 
> Hi Sargun,
> 
> I had asked bunch of questions in previous mail thread to be more
> clear on your requirements but never got any response. It would
> have helped understanding your requirements better.
> 
> How about following patch set which seems to sync only dirty inodes of
> upper belonging to a particular overlayfs instance.
> 
> https://lore.kernel.org/linux-unionfs/20201113065555.147276-1-cgxu519@mykernel.net/
> 
> So if could implement a mount option which ignores fsync but upon
> syncfs, only syncs dirty inodes of that overlayfs instance, it will
> make sure we are not syncing whole of the upper fs. And we could
> do this syncing on unmount of overlayfs and remove dirty file upon
> successful sync.
> 
> Looks like this will be much simpler method and should be able to
> meet your requirements (As long as you are fine with syncing dirty
> upper inodes of this overlay instance on unmount).

This approach also has the advantage error detection is much more granular
and you don't have to throw away container A if there was a writeback
issue in any other unrelated container N sharing same upper.

Thanks
Vivek