linux-kernel - Re: [RFC PATCH v1 00/30] fs: inode->i

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Date:   Tue, 4 Apr 2017 13:53:38 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jan Kara <jack@...e.cz>, Jeff Layton <jlayton@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

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.