linux-kernel - Re: [PATCH RFC] vfs: add a O

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Date:	Wed, 22 Jul 2015 15:47:13 +0200
From:	Pavel Machek <pavel@....cz>
To:	NeilBrown <neilb@...e.com>
Cc:	NeilBrown <neilb@...e.de>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	John Stoffel <john@...ffel.org>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Kevin Easton <kevin@...rana.org>,
	Theodore Ts'o <tytso@....edu>, Sage Weil <sage@...dream.net>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Dave Chinner <david@...morbit.com>,
	Zach Brown <zab@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux API Mailing List <linux-api@...r.kernel.org>
Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag

On Wed 2015-07-15 14:54:56, NeilBrown wrote:
> On Tue, 14 Jul 2015 15:13:00 +0200 Pavel Machek <pavel@....cz> wrote:
> 
> > Hi!
> > 
> > > BTW When you "swap" to a file the mtime doesn't get updated.  No one seems to
> > > complain about that.  I guess it is a rather narrow use-case though.
> > 
> > Actually yes, I'd like to complain.
> > 
> > It was not swap, it was mount -o loop, but I guess that's the same
> > case. Then rsync refused to work on that file... and being on slow ARM
> > system it took me a while to figure out WTF is going on.
> > 
> > So yes, we have problems with mtime, and yes, they matter.
> > 									Pavel
> 
> Odd...
> I assume you mean
>   mount -o loop /some/file  /mountpoint
> 
> and then when you write to the filesystem on /mountpoint the mtime
> of /some/file doesn't get updated?
> I think it should.
>  drivers/block/loop.c uses vfs_iter_write() to write to a file.
>  That calls f_op->write_iter which will typically call
>  generic_file_write_iter() which will call file_update_time() to update
>  the time stamps.

Yes, that. I'm pretty sure I seen it, but it was probably on 2.6.X
kernel... Does it make sense to try to reproduce it on the old kernel?

> What filesystem was /some/file on?

Very probably VFAT.

> I just did some testing on ext4 and it seems to do the right thing
> mtime gets updated.

Yes, I tried here, and it seems to be ok.

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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