Re: [RFC PATCH v7 9/9] vfs: expose STATX_VERSION to userland

Jan Kara
2022-10-18T13:49:21Z

On Tue 18-10-22 06:35:14, Jeff Layton wrote:
> On Tue, 2022-10-18 at 09:14 +1100, Dave Chinner wrote:
> > On Mon, Oct 17, 2022 at 06:57:09AM -0400, Jeff Layton wrote:
> > > Trond is of the opinion that monotonicity is a hard requirement, and
> > > that we should not allow filesystems that can't provide that quality to
> > > report STATX_VERSION at all. His rationale is that one of the main uses
> > > for this is for backup applications, and for those a counter that could
> > > go backward is worse than useless.
> >
> > From the perspective of a backup program doing incremental backups,
> > an inode with a change counter that has a different value to the
> > current backup inventory means the file contains different
> > information than what the current backup inventory holds. Again,
> > snapshots, rollbacks, etc.
> >
> > Therefore, regardless of whether the change counter has gone
> > forwards or backwards, the backup program needs to back up this
> > current version of the file in this backup because it is different
> > to the inventory copy. Hence if the backup program fails to back it
> > up, it will not be creating an exact backup of the user's data at
> > the point in time the backup is run...
> >
> > Hence I don't see that MONOTONIC is a requirement for backup
> > programs - they really do have to be able to handle filesystems that
> > have modifications that move backwards in time as well as forwards...
>
> Rolling backward is not a problem in and of itself. The big issue is
> that after a crash, we can end up with a change attr seen before the
> crash that is now associated with a completely different inode state.
>
> The scenario is something like:
>
> - Change attr for an empty file starts at 1
>
> - Write "A" to file, change attr goes to 2
>
> - Read and statx happens (client sees "A" with change attr 2)
>
> - Crash (before last change is logged to disk)
>
> - Machine reboots, inode is empty, change attr back to 1
>
> - Write "B" to file, change attr goes to 2
>
> - Client stat's file, sees change attr 2 and assumes its cache is
> correct when it isn't (should be "B" not "A" now).
>
> The real danger comes not from the thing going backward, but the fact
> that it can march forward again after going backward, and then the
> client can see two different inode states associated with the same
> change attr value. Jumping all the change attributes forward by a
> significant amount after a crash should avoid this issue.

As Dave pointed out, the problem with change attr having the same value for
a different inode state (after going backwards) holds not only for the
crashes but also for restore from backups, fs snapshots, device snapshots
etc. So relying on change attr only looks a bit fragile. It works for the
common case but the edge cases are awkward and there's no easy way to
detect you are in the edge case.

So I think any implementation caring about data integrity would have to
include something like ctime into the picture anyway. Or we could just
completely give up any idea of monotonicity and on each mount select random
prime P < 2^64 and instead of doing inc when advancing the change
attribute, we'd advance it by P. That makes collisions after restore /
crash fairly unlikely.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR