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SubjectRe: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization
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On Tue, 2017-03-21 at 15:13 -0400, J. Bruce Fields wrote:
> On Tue, Mar 21, 2017 at 02:46:53PM -0400, 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?
>
> No idea. Anyway, I'd like to figure out some reasonable requirement
> that we can document.
>
> >
> > > - if I write to a file, a simultaneous reader should see either
> > > (old data, old i_version) or (new data, new i_version), not a
> > > combination of the two.
> > > - ditto for metadata modifications.
> > > - the same should be true if there's a crash.
> > >
> > > (If that's not possible, then I think we could live with a brief window
> > > of (new data, old i_version) as long as it doesn't persist beyond sync?)
> > >
> > > > That said, I suppose it is possible for us to bump the counter, hand
> > > > that new counter value out to a NFS client and then the box crashes
> > > > before it makes it to the journal.
> > > >
> > > > Not sure how big a problem that really is.
> > >
> > > The other case I was wondering about may have been unclear. Represent
> > > the state of a file by a (data, i_version) pair. Say:
> > >
> > > - file is modified from (F, V) to (F', V+1).
> > > - client reads and caches (F', V+1).
> > > - server crashes before writeback, so disk still has (F, V).
> > > - after restart, someone else modifies file to (F'', V+1).
> > > - original client revalidates its cache, sees V+1, concludes
> > > file data is still F'.
> > >
> > > This may not cause a real problem for clients depending only on
> > > traditional NFS close-to-open semantics.
> > >
> > >
> >
> > No, I think that is a legitimate problem.
> >
> > That said, after F'', the mtime would almost certainly be different
> > from the time after F', and that would likely be enough to prevent
> > confusion in NFS clients.
>
> Oh, good point. So, may be worth saying that anyone wanting to make
> sense of these across reboot should compare times as well (maybe that
> should be in nfs rfc's too). I think that should be ctime not mtime,
> though?
>

Yes, it might be worth a mention there. IIRC, it does mention that you
shouldn't just look at a single attribute for cache validation
purposes, but the wording is a bit vague. I can't find the section at
the moment though.

The more I think about it though, simply ensuring that we don't publish
a new change attr until the inode update has hit the journal may be
the best we can do. I'd have to think about how to implement that
though.
--
Jeff Layton <jlayton@redhat.com>

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