Re: [PATCH v8 0/5] fs: multigrain timestamps for XFS's change_cookie

[Amir Goldstein]

On Sat, Sep 23, 2023 at 1:46 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> On Sat, 2023-09-23 at 10:15 +0300, Amir Goldstein wrote:
> > On Fri, Sep 22, 2023 at 8:15 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> > >
> > > My initial goal was to implement multigrain timestamps on most major
> > > filesystems, so we could present them to userland, and use them for
> > > NFSv3, etc.
> > >
> > > With the current implementation however, we can't guarantee that a file
> > > with a coarse grained timestamp modified after one with a fine grained
> > > timestamp will always appear to have a later value. This could confuse
> > > some programs like make, rsync, find, etc. that depend on strict
> > > ordering requirements for timestamps.
> > >
> > > The goal of this version is more modest: fix XFS' change attribute.
> > > XFS's change attribute is bumped on atime updates in addition to other
> > > deliberate changes. This makes it unsuitable for export via nfsd.
> > >
> > > Jan Kara suggested keeping this functionality internal-only for now and
> > > plumbing the fine grained timestamps through getattr [1]. This set takes
> > > a slightly different approach and has XFS use the fine-grained attr to
> > > fake up STATX_CHANGE_COOKIE in its getattr routine itself.
> > >
> > > While we keep fine-grained timestamps in struct inode, when presenting
> > > the timestamps via getattr, we truncate them at a granularity of number
> > > of ns per jiffy,
> >
> > That's not good, because user explicitly set granular mtime would be
> > truncated too and booting with different kernels (HZ) would change
> > the observed timestamps of files.
> >
>
> Thinking about this some more, I think the first problem is easily
> addressable:
>
> The ctime isn't explicitly settable and with this set, we're already not
> truncating the atime. We haven't used any of the extra bits in the mtime
> yet, so we could just carve out a flag in there that says "this mtime
> was explicitly set and shouldn't be truncated before presentation".
>
> The second problem (booting to older kernel) is a bit tougher to deal
> with however. I'll have to think about that one a bit more.

There is a straightforward solution to both these problems -
opt in with a mount option to truncate user visible timestamps to 100ns
(and not jiffy) resolution as Linus is trying to promote.

Thanks,
Amir.