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

[Vivek Goyal <vgoyal@redhat.com>]

On Sat, Nov 07, 2020 at 11:35:04AM +0200, Amir Goldstein wrote:
> 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.

Hi Amir,

I am wondering why "sync -f upper/" is not sufficient and why full sync
is required.

Vivek