Date:   Tue, 19 Sep 2023 12:31:08 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Bruno Haible <bruno@...sp.org>, Jan Kara <jack@...e.cz>,
        Xi Ruoyao <xry111@...uxfromscratch.org>, bug-gnulib@....org
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Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps

On Tue, 2023-09-19 at 16:52 +0200, Bruno Haible wrote:
> Jeff Layton wrote:
> > I'm not sure what we can do for this test. The nap() function is making
> > an assumption that the timestamp granularity will be constant, and that
> > isn't necessarily the case now.
> 
> This is only of secondary importance, because the scenario by Jan Kara
> shows a much more fundamental breakage:
> 
> > > The ultimate problem is that a sequence like:
> > > 
> > > write(f1)
> > > stat(f2)
> > > write(f2)
> > > stat(f2)
> > > write(f1)
> > > stat(f1)
> > > 
> > > can result in f1 timestamp to be (slightly) lower than the final f2
> > > timestamp because the second write to f1 didn't bother updating the
> > > timestamp. That can indeed be a bit confusing to programs if they compare
> > > timestamps between two files. Jeff?
> > > 
> > 
> > Basically yes.
> 
> f1 was last written to *after* f2 was last written to. If the timestamp of f1
> is then lower than the timestamp of f2, timestamps are fundamentally broken.
> 
> Many things in user-space depend on timestamps, such as build system
> centered around 'make', but also 'find ... -newer ...'.
> 


What does breakage with make look like in this situation? The "fuzz"
here is going to be on the order of a jiffy. The typical case for make
timestamp comparisons is comparing source files vs. a build target. If
those are being written nearly simultaneously, then that could be an
issue, but is that a typical behavior? It seems like it would be hard to
rely on that anyway, esp. given filesystems like NFS that can do lazy
writeback.

One of the operating principles with this series is that timestamps can
be of varying granularity between different files. Note that Linux
already violates this assumption when you're working across filesystems
of different types.

As to potential fixes if this is a real problem:

I don't really want to put this behind a mount or mkfs option (a'la
relatime, etc.), but that is one possibility.

I wonder if it would be feasible to just advance the coarse-grained
current_time whenever we end up updating a ctime with a fine-grained
timestamp? It might produce some inode write amplification. Files that
were written within the same jiffy could see more inode transactions
logged, but that still might not be _too_ awful.

I'll keep thinking about it for now.
-- 
Jeff Layton <jlayton@...nel.org>

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