Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2023-11-01 at 10:10 -1000, Linus Torvalds wrote:
> On Wed, 1 Nov 2023 at 00:16, Jan Kara <jack@suse.cz> wrote:
> > 
> > OK, but is this compatible with the current XFS behavior? AFAICS currently
> > XFS sets sb->s_time_gran to 1 so timestamps currently stored on disk will
> > have some mostly random garbage in low bits of the ctime.
> 
> I really *really* don't think we can use ctime as a "i_version"
> replacement. The whole fine-granularity patches were well-intentioned,
> but I do think they were broken.
> 

I have to take some issue here. I still the basic concept is sound. The
original implementation was flawed but I think I have a scheme that
could address the problems with the multigrain series.

That said, everyone seems to be haring off after other solutions. I
don't much care which one we end up with, as long as the problem gets
fixed.

> Note that we can't use ctime as a "i_version" replacement for other
> reasons too - you have filesystems like FAT - which people do want to
> export - that have a single-second (or is it 2s?) granularity in
> reality, even though they report a 1ns value in s_time_gran.
> 
> But here's a suggestion that people may hate, but that might just work
> in practice:
> 
>  - get rid of i_version entirely
> 
>  - use the "known good" part of ctime as the upper bits of the change
> counter (and by "known good" I mean tv_sec - or possibly even "tv_sec
> / 2" if that dim FAT memory of mine is right)
> 
>  - make the rule be that ctime is *never* updated for atime updates
> (maybe that's already true, I didn't check - maybe it needs a new
> mount flag for nfsd)
> 
>  - have a per-inode in-memory and vfs-internal (entirely invisible to
> filesystems) "ctime modification counter" that is *NOT* a timestamp,
> and is *NOT* i_version
> 
>  - make the rule be that the "ctime modification counter" is always
> zero, *EXCEPT* if
>     (a) I_VERSION_QUERIED is set
>    AND
>     (b) the ctime modification doesn't modify the "known good" part of ctime
> 
> so how the "statx change cookie" ends up being "high bits tv_sec of
> ctime, low bits ctime modification cookie", and the end result of that
> is:
> 
>  - if all the reads happen after the last write (common case), then
> the low bits will be zero, because I_VERSION_QUERIED wasn't set when
> ctime was modified
> 
>  - if you do a write *after* a modification, the ctime cookie is
> guaranteed to change, because either the known good (sec/2sec) part of
> ctime is new, *or* the counter gets updated
> 
>  - if the nfs server reboots, the in-memory counter will be cleared
> again, and so the change cookie will cause client cache invalidations,
> but *only* for those "ctime changed in the same second _after_
> somebody did a read".
> 
>  - any long-time caches of files that don't get modified are all fine,
> because they will have those low bits zero and depend on just the
> stable part of ctime that works across filesystems. So there should be
> no nasty thundering herd issues on long-lived caches on lots of
> clients if the server reboots, or atime updates every 24 hours or
> anything like that.
> 
> and note that *NONE* of this requires any filesystem involvement
> (except for the rule of "no atime changes ever impact ctime", which
> may or may not already be true).
> 
> The filesystem does *not* know about that modification counter,
> there's no new on-disk stable information.
> 
> It's entirely possible that I'm missing something obvious, but the
> above sounds to me like the only time you'd have stale invalidations
> is really the (unusual) case of having writes after cached reads, and
> then a reboot.
> 
> We'd get rid of "inode_maybe_inc_iversion()" entirely, and instead
> replace it with logic in inode_set_ctime_current() that basically does
> 
>  - if the stable part of ctime changes, clear the new 32-bit counter
> 
>  - if I_VERSION_QUERIED isn't set, clear the new 32-bit counter
> 
>  - otherwise, increment the new 32-bit counter
> 
> and then the STATX_CHANGE_COOKIE code basically just returns
> 
>    (stable part of ctime << 32) + new 32-bit counter
> 
> (and again, the "stable part of ctime" is either just tv_sec, or it's
> "tv_sec >> 1" or whatever).
> 
> The above does not expose *any* changes to timestamps to users, and
> should work across a wide variety of filesystems, without requiring
> any special code from the filesystem itself.
> 
> And now please all jump on me and say "No, Linus, that won't work, because XYZ".
> 
> Because it is *entirely* possible that I missed something truly
> fundamental, and the above is completely broken for some obvious
> reason that I just didn't think of.
> 

Yeah, I think this scheme is problematic for the reasons Trond pointed
out. I also don't quite see the advantage of this over what Dave Chinner
is proposing (using low-order bits of the ctime nsec field to hold a
change counter).
-- 
Jeff Layton <jlayton@kernel.org>