On Fri, Nov 6, 2020 at 9:43 PM Giuseppe Scrivano <gscrivan@xxxxxxxxxx> wrote: > > Vivek Goyal <vgoyal@xxxxxxxxxx> 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.
