Message-id: <166328063547.15759.12797959071252871549@noble.neil.brown.name>
Date:   Fri, 16 Sep 2022 08:23:55 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Jeff Layton" <jlayton@...nel.org>
Cc:     "Trond Myklebust" <trondmy@...merspace.com>,
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Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Fri, 16 Sep 2022, Jeff Layton wrote:
> On Thu, 2022-09-15 at 19:03 +0000, Trond Myklebust wrote:
> > On Thu, 2022-09-15 at 14:11 -0400, Jeff Layton wrote:
> > > On Thu, 2022-09-15 at 17:49 +0000, Trond Myklebust wrote:
> > > > On Thu, 2022-09-15 at 12:45 -0400, Jeff Layton wrote:
> > > > > On Thu, 2022-09-15 at 15:08 +0000, Trond Myklebust wrote:
> > > > > > On Thu, 2022-09-15 at 10:06 -0400, J. Bruce Fields wrote:
> > > > > > > On Tue, Sep 13, 2022 at 09:14:32AM +1000, NeilBrown wrote:
> > > > > > > > On Mon, 12 Sep 2022, J. Bruce Fields wrote:
> > > > > > > > > On Sun, Sep 11, 2022 at 08:13:11AM +1000, NeilBrown
> > > > > > > > > wrote:
> > > > > > > > > > On Fri, 09 Sep 2022, Jeff Layton wrote:
> > > > > > > > > > > 
> > > > > > > > > > > The machine crashes and comes back up, and we get a
> > > > > > > > > > > query
> > > > > > > > > > > for
> > > > > > > > > > > i_version
> > > > > > > > > > > and it comes back as X. Fine, it's an old version.
> > > > > > > > > > > Now
> > > > > > > > > > > there
> > > > > > > > > > > is a write.
> > > > > > > > > > > What do we do to ensure that the new value doesn't
> > > > > > > > > > > collide
> > > > > > > > > > > with X+1? 
> > > > > > > > > > 
> > > > > > > > > > (I missed this bit in my earlier reply..)
> > > > > > > > > > 
> > > > > > > > > > How is it "Fine" to see an old version?
> > > > > > > > > > The file could have changed without the version
> > > > > > > > > > changing.
> > > > > > > > > > And I thought one of the goals of the crash-count was
> > > > > > > > > > to be
> > > > > > > > > > able to
> > > > > > > > > > provide a monotonic change id.
> > > > > > > > > 
> > > > > > > > > I was still mainly thinking about how to provide reliable
> > > > > > > > > close-
> > > > > > > > > to-open
> > > > > > > > > semantics between NFS clients.  In the case the writer
> > > > > > > > > was an
> > > > > > > > > NFS
> > > > > > > > > client, it wasn't done writing (or it would have
> > > > > > > > > COMMITted),
> > > > > > > > > so
> > > > > > > > > those
> > > > > > > > > writes will come in and bump the change attribute soon,
> > > > > > > > > and
> > > > > > > > > as
> > > > > > > > > long as
> > > > > > > > > we avoid the small chance of reusing an old change
> > > > > > > > > attribute,
> > > > > > > > > we're OK,
> > > > > > > > > and I think it'd even still be OK to advertise
> > > > > > > > > CHANGE_TYPE_IS_MONOTONIC_INCR.
> > > > > > > > 
> > > > > > > > You seem to be assuming that the client doesn't crash at
> > > > > > > > the
> > > > > > > > same
> > > > > > > > time
> > > > > > > > as the server (maybe they are both VMs on a host that lost
> > > > > > > > power...)
> > > > > > > > 
> > > > > > > > If client A reads and caches, client B writes, the server
> > > > > > > > crashes
> > > > > > > > after
> > > > > > > > writing some data (to already allocated space so no inode
> > > > > > > > update
> > > > > > > > needed)
> > > > > > > > but before writing the new i_version, then client B
> > > > > > > > crashes.
> > > > > > > > When server comes back the i_version will be unchanged but
> > > > > > > > the
> > > > > > > > data
> > > > > > > > has
> > > > > > > > changed.  Client A will cache old data indefinitely...
> > > > > > > 
> > > > > > > I guess I assume that if all we're promising is close-to-
> > > > > > > open,
> > > > > > > then a
> > > > > > > client isn't allowed to trust its cache in that situation. 
> > > > > > > Maybe
> > > > > > > that's
> > > > > > > an overly draconian interpretation of close-to-open.
> > > > > > > 
> > > > > > > Also, I'm trying to think about how to improve things
> > > > > > > incrementally.
> > > > > > > Incorporating something like a crash count into the on-disk
> > > > > > > i_version
> > > > > > > fixes some cases without introducing any new ones or
> > > > > > > regressing
> > > > > > > performance after a crash.
> > > > > > > 
> > > > > > > If we subsequently wanted to close those remaining holes, I
> > > > > > > think
> > > > > > > we'd
> > > > > > > need the change attribute increment to be seen as atomic with
> > > > > > > respect
> > > > > > > to
> > > > > > > its associated change, both to clients and (separately) on
> > > > > > > disk. 
> > > > > > > (That
> > > > > > > would still allow the change attribute to go backwards after
> > > > > > > a
> > > > > > > crash,
> > > > > > > to
> > > > > > > the value it held as of the on-disk state of the file.  I
> > > > > > > think
> > > > > > > clients
> > > > > > > should be able to deal with that case.)
> > > > > > > 
> > > > > > > But, I don't know, maybe a bigger hammer would be OK:
> > > > > > > 
> > > > > > 
> > > > > > If you're not going to meet the minimum bar of data integrity,
> > > > > > then
> > > > > > this whole exercise is just a massive waste of everyone's time.
> > > > > > The
> > > > > > answer then going forward is just to recommend never using
> > > > > > Linux as
> > > > > > an
> > > > > > NFS server. Makes my life much easier, because I no longer have
> > > > > > to
> > > > > > debug any of the issues.
> > > > > > 
> > > > > > 
> > > > > 
> > > > > To be clear, you believe any scheme that would allow the client
> > > > > to
> > > > > see
> > > > > an old change attr after a crash is insufficient?
> > > > > 
> > > > 
> > > > Correct. If a NFSv4 client or userspace application cannot trust
> > > > that
> > > > it will always see a change to the change attribute value when the
> > > > file
> > > > data changes, then you will eventually see data corruption due to
> > > > the
> > > > cached data no longer matching the stored data.
> > > > 
> > > > A false positive update of the change attribute (i.e. a case where
> > > > the
> > > > change attribute changes despite the data/metadata staying the
> > > > same) is
> > > > not desirable because it causes performance issues, but false
> > > > negatives
> > > > are far worse because they mean your data backup, cache, etc... are
> > > > not
> > > > consistent. Applications that have strong consistency requirements
> > > > will
> > > > have no option but to revalidate by always reading the entire file
> > > > data
> > > > + metadata.
> > > > 
> > > > > The only way I can see to fix that (at least with only a crash
> > > > > counter)
> > > > > would be to factor it in at presentation time like Neil
> > > > > suggested.
> > > > > Basically we'd just mask off the top 16 bits and plop the crash
> > > > > counter
> > > > > in there before presenting it.
> > > > > 
> > > > > In principle, I suppose we could do that at the nfsd level as
> > > > > well
> > > > > (and
> > > > > that might be the simplest way to fix this). We probably wouldn't
> > > > > be
> > > > > able to advertise a change attr type of MONOTONIC with this
> > > > > scheme
> > > > > though.
> > > > 
> > > > Why would you want to limit the crash counter to 16 bits?
> > > > 
> > > 
> > > To leave more room for the "real" counter. Otherwise, an inode that
> > > gets
> > > frequent writes after a long period of no crashes could experience
> > > the
> > > counter wrap.
> > > 
> > > IOW, we have 63 bits to play with. Whatever part we dedicate to the
> > > crash counter will not be available for the actual version counter.
> > > 
> > > I'm proposing a 16+47+1 split, but I'm happy to hear arguments for a
> > > different one.
> > 
> > 
> > What is the expectation when you have an unclean shutdown or crash? Do
> > all change attribute values get updated to reflect the new crash
> > counter value, or only some?
> > 
> > If the answer is that 'all values change', then why store the crash
> > counter in the inode at all? Why not just add it as an offset when
> > you're generating the user-visible change attribute?
> > 
> > i.e. statx.change_attr = inode->i_version + (crash counter * offset)
> > 
> > (where offset is chosen to be larger than the max number of inode-
> > > i_version updates that could get lost by an inode in a crash).
> > 
> > Presumably that offset could be significantly smaller than 2^63...
> > 
> 
> 
> Yes, if we plan to ensure that all the change attrs change after a
> crash, we can do that.
> 
> So what would make sense for an offset? Maybe 2**12? One would hope that
> there wouldn't be more than 4k increments before one of them made it to
> disk. OTOH, maybe that can happen with teeny-tiny writes.

Leave it up the to filesystem to decide.  The VFS and/or NFSD should
have not have part in calculating the i_version.  It should be entirely
in the filesystem - though support code could be provided if common
patterns exist across filesystems.

A filesystem *could* decide to ensure the on-disk i_version is updated
when the difference between in-memory and on-disk reaches X/2, and add X
after an unclean restart.  Or it could just choose a large X and hope.
Or it could do something else that neither of us has thought of.  But
PLEASE leave the filesystem in control, do not make it fit with our
pre-conceived ideas of what would be easy for it.

> 
> If we want to leave this up to the filesystem, I guess we could just add
> a new struct super_block.s_version_offset field and let the filesystem
> precompute that value and set it at mount time. Then we can just add
> that in after querying i_version.

If we are leaving "this up to the filesystem", the we don't add anything
to struct super_block and we don't add anything "in after querying
i_version".  Rather, we "leave this up to the filesystem" and use
exactly the i_version that the filesystem provides.  We only provide
advice as to minimum requirements, preferred behaviours, and possible
implementation suggestions.

NeilBrown


> -- 
> Jeff Layton <jlayton@...nel.org>
> 

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