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    SubjectRe: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization
    On Wed 29-03-17 13:54:31, Jeff Layton wrote:
    > On Wed, 2017-03-29 at 13:15 +0200, Jan Kara wrote:
    > > On Tue 21-03-17 14:46:53, Jeff Layton wrote:
    > > > On Tue, 2017-03-21 at 14:30 -0400, J. Bruce Fields wrote:
    > > > > On Tue, Mar 21, 2017 at 01:23:24PM -0400, Jeff Layton wrote:
    > > > > > On Tue, 2017-03-21 at 12:30 -0400, J. Bruce Fields wrote:
    > > > > > > - It's durable; the above comparison still works if there were reboots
    > > > > > > between the two i_version checks.
    > > > > > > - I don't know how realistic this is--we may need to figure out
    > > > > > > if there's a weaker guarantee that's still useful. Do
    > > > > > > filesystems actually make ctime/mtime/i_version changes
    > > > > > > atomically with the changes that caused them? What if a
    > > > > > > change attribute is exposed to an NFS client but doesn't make
    > > > > > > it to disk, and then that value is reused after reboot?
    > > > > > >
    > > > > >
    > > > > > Yeah, there could be atomicity there. If we bump i_version, we'll mark
    > > > > > the inode dirty and I think that will end up with the new i_version at
    > > > > > least being journalled before __mark_inode_dirty returns.
    > > > >
    > > > > So you think the filesystem can provide the atomicity? In more detail:
    > > > >
    > > >
    > > > Sorry, I hit send too quickly. That should have read:
    > > >
    > > > "Yeah, there could be atomicity issues there."
    > > >
    > > > I think providing that level of atomicity may be difficult, though
    > > > maybe there's some way to make the querying of i_version block until
    > > > the inode update has been journalled?
    > >
    > > Just to complement what Dave said from ext4 side - similarly as with XFS
    > > ext4 doesn't guarantee atomicity unless fsync() has completed on the file.
    > > Until that you can see arbitrary combination of data & i_version after the
    > > crash. We do take care to keep data and metadata in sync only when there
    > > are security implications to that (like exposing uninitialized disk blocks)
    > > and if not, we are as lazy as we can to improve performance...
    > >
    > >
    >
    > Yeah, I think what we'll have to do here is ensure that those
    > filesystems do an fsync prior to reporting the i_version getattr
    > codepath. It's not pretty, but I don't see a real alternative.

    Hum, so are we fine if i_version just changes (increases) for all inodes
    after a server crash? If I understand its use right, it would mean
    invalidation of all client's caches but that is not such a big deal given
    how frequent server crashes should be, right?

    Because if above is acceptable we could make reported i_version to be a sum
    of "superblock crash counter" and "inode i_version". We increment
    "superblock crash counter" whenever we detect unclean filesystem shutdown.
    That way after a crash we are guaranteed each inode will report new
    i_version (the sum would probably have to look like "superblock crash
    counter" * 65536 + "inode i_version" so that we avoid reusing possible
    i_version numbers we gave away but did not write to disk but still...).
    Thoughts?

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

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