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 Regards, Giuseppe
