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Date:   Tue, 19 Sep 2023 13:04:57 +0200
From:   Jan Kara <jack@suse.cz>
To:     Xi Ruoyao <xry111@linuxfromscratch.org>
Cc:     Jeff Layton <jlayton@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Jan Kara <jack@suse.cz>, bug-gnulib@gnu.org
Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps
Message-ID: <20230919110457.7fnmzo4nqsi43yqq@quack3>
References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org>
 <20230807-mgctime-v7-12-d1dec143a704@kernel.org>
 <bf0524debb976627693e12ad23690094e4514303.camel@linuxfromscratch.org>
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On Tue 19-09-23 15:05:24, Xi Ruoyao wrote:
> On Mon, 2023-08-07 at 15:38 -0400, Jeff Layton wrote:
> > Enable multigrain timestamps, which should ensure that there is an
> > apparent change to the timestamp whenever it has been written after
> > being actively observed via getattr.
> > 
> > For ext4, we only need to enable the FS_MGTIME flag.
> 
> Hi Jeff,
> 
> This patch causes a gnulib test failure:
> 
> $ ~/sources/lfs/grep-3.11/gnulib-tests/test-stat-time
> test-stat-time.c:141: assertion 'statinfo[0].st_mtime < statinfo[2].st_mtime || (statinfo[0].st_mtime == statinfo[2].st_mtime && (get_stat_mtime_ns (&statinfo[0]) < get_stat_mtime_ns (&statinfo[2])))' failed
> Aborted (core dumped)
> 
> The source code of the test:
> https://git.savannah.gnu.org/cgit/gnulib.git/tree/tests/test-stat-time.c
> 
> Is this an expected change?

Kind of yes. The test first tries to estimate filesystem timestamp
granularity in nap() function - due to this patch, the detected granularity
will likely be 1 ns so effectively all the test calls will happen
immediately one after another. But we don't bother setting the timestamps
with more than 1 jiffy (usually 4 ms) precision unless we think someone is
watching. So as a result timestamps of all stamp1 and stamp2 files are
going to be equal which makes the test fail.

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?

								Honza


> > Acked-by: Theodore Ts'o <tytso@mit.edu>
> > Reviewed-by: Jan Kara <jack@suse.cz>
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> > ?fs/ext4/super.c | 2 +-
> > ?1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index b54c70e1a74e..cb1ff47af156 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -7279,7 +7279,7 @@ static struct file_system_type ext4_fs_type = {
> > ?	.init_fs_context	= ext4_init_fs_context,
> > ?	.parameters		= ext4_param_specs,
> > ?	.kill_sb		= kill_block_super,
> > -	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
> > +	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP |
> > FS_MGTIME,
> > ?};
> > ?MODULE_ALIAS_FS("ext4");
> > ?
> > 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR