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Message-ID: <c8315110-4684-9b83-d6c5-751647037623@cs.ucla.edu>
Date:   Tue, 19 Sep 2023 13:10:47 -0700
From:   Paul Eggert <eggert@...ucla.edu>
To:     Jeff Layton <jlayton@...nel.org>, Bruno Haible <bruno@...sp.org>,
        Jan Kara <jack@...e.cz>,
        Xi Ruoyao <xry111@...uxfromscratch.org>, bug-gnulib@....org
Cc:     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>,
        Bo b 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>,
        Mark Fasheh <mark@...heh.com>,
        Joel Becker <jlbec@...lplan.org>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Mike Marshall <hubcap@...ibond.com>,
        Martin Brandenburg <martin@...ibond.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
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        Paulo Alcantara <pc@...guebit.com>,
        Ronnie Sahlberg <ronniesahlberg@...il.com>,
        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 <l@...il.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Benjamin Coddington <bcodding@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps

On 2023-09-19 09:31, Jeff Layton wrote:
> 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?

I vaguely remember running into problems with 'make' a while ago 
(perhaps with a BSDish system) when filesystem timestamps were 
arbitrarily truncated in some cases but not others. These files would 
look older than they really were, so 'make' would think they were 
up-to-date when they weren't, and 'make' would omit actions that it 
should have done, thus screwing up the build.

File timestamps can be close together with 'make -j' on fast hosts. 
Sometimes a shell script (or 'make' itself) will run 'make', then modify 
a file F, then immediately run 'make' again; the latter 'make' won't 
work if F's timestamp is mistakenly older than targets that depend on it.

Although 'make'-like apps are the biggest canaries in this coal mine, 
the issue also affects 'find -newer' (as Bruno mentioned), 'rsync -u', 
'mv -u', 'tar -u', Emacs file-newer-than-file-p, and surely many other 
places. For example, any app that creates a timestamp file, then backs 
up all files newer than that file, would be at risk.


> 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?

Wouldn't this need to be done globally, that is, not just on a per-file 
or per-filesystem basis? If so, I don't see how we'd avoid locking 
performance issues.


PS. Although I'm no expert in the Linux inode code I hope you don't mind 
my asking a question about this part of inode_set_ctime_current:

	/*
	 * If we've recently updated with a fine-grained timestamp,
	 * then the coarse-grained one may still be earlier than the
	 * existing ctime. Just keep the existing value if so.
	 */
	ctime.tv_sec = inode->__i_ctime.tv_sec;
	if (timespec64_compare(&ctime, &now) > 0)
		return ctime;

Suppose root used clock_settime to set the clock backwards. Won't this 
code incorrectly refuse to update the file's timestamp afterwards? That 
is, shouldn't the last line be "goto fine_grained;" rather than "return 
ctime;", with the comment changed from "keep the existing value" to "use 
a fine-grained value"?

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