Re: [PATCH v4] overlay: Implement volatile-specific fsync error behaviour

[Miklos Szeredi <miklos@szeredi.hu>]

On Fri, Jan 8, 2021 at 1:10 AM Sargun Dhillon <sargun@sargun.me> wrote:
>
> Overlayfs's volatile option allows the user to bypass all forced sync calls
> to the upperdir filesystem. This comes at the cost of safety. We can never
> ensure that the user's data is intact, but we can make a best effort to
> expose whether or not the data is likely to be in a bad state.
>
> The best way to handle this in the time being is that if an overlayfs's
> upperdir experiences an error after a volatile mount occurs, that error
> will be returned on fsync, fdatasync, sync, and syncfs. This is
> contradictory to the traditional behaviour of VFS which fails the call
> once, and only raises an error if a subsequent fsync error has occurred,
> and been raised by the filesystem.
>
> One awkward aspect of the patch is that we have to manually set the
> superblock's errseq_t after the sync_fs callback as opposed to just
> returning an error from syncfs. This is because the call chain looks
> something like this:
>
> sys_syncfs ->
>         sync_filesystem ->
>                 __sync_filesystem ->
>                         /* The return value is ignored here
>                         sb->s_op->sync_fs(sb)
>                         _sync_blockdev
>                 /* Where the VFS fetches the error to raise to userspace */
>                 errseq_check_and_advance
>
> Because of this we call errseq_set every time the sync_fs callback occurs.
> Due to the nature of this seen / unseen dichotomy, if the upperdir is an
> inconsistent state at the initial mount time, overlayfs will refuse to
> mount, as overlayfs cannot get a snapshot of the upperdir's errseq that
> will increment on error until the user calls syncfs.

Thanks, this makes sense.  Queued for v4.11.

Miklos