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Message-ID: <20230919110457.7fnmzo4nqsi43yqq@quack3>
Date:   Tue, 19 Sep 2023 13:04:57 +0200
From:   Jan Kara <jack@...e.cz>
To:     Xi Ruoyao <xry111@...uxfromscratch.org>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Eric Van Hensbergen <ericvh@...nel.org>,
        Latchesar Ionkov <lucho@...kov.net>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Christian Schoenebeck <linux_oss@...debyte.com>,
        David Howells <dhowells@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Xiubo Li <xiubli@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        Jan Harkes <jaharkes@...cmu.edu>, coda@...cmu.edu,
        Tyler Hicks <code@...icks.com>, Gao Xiang <xiang@...nel.org>,
        Chao Yu <chao@...nel.org>, Yue Hu <huyue2@...lpad.com>,
        Jeffle Xu <jefflexu@...ux.alibaba.com>,
        Namjae Jeon <linkinjeon@...nel.org>,
        Sungjong Seo <sj1557.seo@...sung.com>,
        Jan Kara <jack@...e.com>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Miklos Szeredi <miklos@...redi.hu>,
        Bob Peterson <rpeterso@...hat.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Konstantin Komarov <almaz.alexandrovich@...agon-software.com>,
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        Joel Becker <jlbec@...lplan.org>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
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        Martin Brandenburg <martin@...ibond.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
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        Steve French <sfrench@...ba.org>,
        Paulo Alcantara <pc@...guebit.com>,
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        Shyam Prasad N <sprasad@...rosoft.com>,
        Tom Talpey <tom@...pey.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Richard Weinberger <richard@....at>,
        Hans de Goede <hdegoede@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Benjamin Coddington <bcodding@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        v9fs@...ts.linux.dev, linux-afs@...ts.infradead.org,
        linux-btrfs@...r.kernel.org, ceph-devel@...r.kernel.org,
        codalist@...a.cs.cmu.edu, ecryptfs@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-nfs@...r.kernel.org, ntfs3@...ts.linux.dev,
        ocfs2-devel@...ts.linux.dev, devel@...ts.orangefs.org,
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        linux-unionfs@...r.kernel.org, linux-xfs@...r.kernel.org,
        Jan Kara <jack@...e.cz>, bug-gnulib@....org
Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps

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@....edu>
> > Reviewed-by: Jan Kara <jack@...e.cz>
> > Signed-off-by: Jeff Layton <jlayton@...nel.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@...e.com>
SUSE Labs, CR

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