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".
>

I thought about this option too.
But note that the "mtime was explicitly set" flag needs
to be persisted to disk so you cannot store it in the high nsec bits.
At least XFS won't store those bits if you use them - they have to
be translated to an XFS inode flag and I don't know if changing
XFS on-disk format was on your wish list.

Thanks,
Amir.