Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

[J. Bruce Fields]

On Tue, Apr 04, 2017 at 10:34:14PM +1000, Dave Chinner wrote:
> On Mon, Apr 03, 2017 at 04:00:55PM +0200, Jan Kara wrote:
> > What filesystems can or cannot easily do obviously differs. Ext4 has a
> > recovery flag set in superblock on RW mount/remount and cleared on
> > umount/RO remount.
> 
> Even this doesn't help. A recent bug that was reported to the XFS
> list - turns out that systemd can't remount-ro the root
> filesystem sucessfully on shutdown because there are open write fds
> on the root filesystem when it attempts the remount. So it just
> reboots without a remount-ro.

I'd certainly rather not invalidate caches on *every* boot.

On the other hand, if the only cases involve the root filesystem, then
from the point of view of NFS, we probably don't care much.

> > This flag being set on mount would imply incrementing the crash
> > counter. It should be pretty easy for each filesystem to implement
> > such flag and the counter but I agree it requires an on-disk
> > format change.
> 
> Yup, anything we want that is persistent and consistent across
> filesystems will need on-disk format changes. Hence we need a solid
> specification first, not to mention tests to validate correct
> behaviour across all filesystems in xfstests...

For xfstests we'll need a way to get i_version (patch it into statx, I
guess?).  Ideally we'd have a way to test behavior across crashes too,
any advice there?

--b.