Re: [PATCH v7] overlayfs: Provide a mount option "volatile" to skip sync

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Nov 6, 2020 at 9:43 PM Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>
> Vivek Goyal <vgoyal@redhat.com> writes:
>
> > On Fri, Nov 06, 2020 at 09:58:39AM -0800, Sargun Dhillon wrote:
> >
> > [..]
> >> There is some slightly confusing behaviour here [I realize this
> >> behaviour is as intended]:
> >>
> >> (root) ~ # mount -t overlay -o
> >> volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir
> >> none /mnt/foo
> >> (root) ~ # umount /mnt/foo
> >> (root) ~ # mount -t overlay -o
> >> volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir
> >> none /mnt/foo
> >> mount: /mnt/foo: wrong fs type, bad option, bad superblock on none,
> >> missing codepage or helper program, or other error.
> >>
> >> From my understanding, the dirty flag should only be a problem if the
> >> existing overlayfs is unmounted uncleanly. Docker does
> >> this (mount, and re-mounts) during startup time because it writes some
> >> files to the overlayfs. I think that we should harden
> >> the volatile check slightly, and make it so that within the same boot,
> >> it's not a problem, and having to have the user clear
> >> the workdir every time is a pain. In addition, the semantics of the
> >> volatile patch itself do not appear to be such that they
> >> would break mounts during the same boot / mount of upperdir -- as
> >> overlayfs does not defer any writes in itself, and it's
> >> only that it's short-circuiting writes to the upperdir.
> >
> > umount does a sync normally and with "volatile" overlayfs skips that
> > sync. So a successful unmount does not mean that file got synced
> > to backing store. It is possible, after umount, system crashed
> > and after reboot, user tried to mount upper which is corrupted
> > now and overlay will not detect it.
> >
> > You seem to be asking for an alternate option where we disable
> > fsync() but not syncfs. In that case sync on umount will still
> > be done. And that means a successful umount should mean upper
> > is fine and it could automatically remove incomapt dir upon
> > umount.
>
> could this be handled in user space?  It should still be possible to do
> the equivalent of:
>
> # sync -f /root/upperdir
> # rm -rf /root/workdir/incompat/volatile
>

FWIW, the sync -f command above is
1. Not needed when re-mounting overlayfs as volatile
2. Not enough when re-mounting overlayfs as non-volatile

In the latter case, a full sync (no -f) is required.

Handling this is userspace is the preferred option IMO,
but if there is an *appealing* reason to allow opportunistic
volatile overlayfs re-mount as long as the upperdir inode
is in cache (userspace can make sure of that), then
all I am saying is that it is possible and not terribly hard.

Thanks,
Amir.