linux-ext4 - Re: [PATCH v3 3/5] mm: Notify filesystems when it's time to apply a deferred cmtime update

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Date:	Tue, 20 Aug 2013 09:42:34 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jan Kara <jack@...e.cz>
Cc:	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Dave Hansen <dave.hansen@...ux.intel.com>, xfs@....sgi.com,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v3 3/5] mm: Notify filesystems when it's time to apply a
 deferred cmtime update

On Tue, Aug 20, 2013 at 9:00 AM, Jan Kara <jack@...e.cz> wrote:
> On Mon 19-08-13 21:14:44, Andy Lutomirski wrote:
>> >> I could require ->writepages *and* ->flush_cmtime to handle the time
>> >> update, but that would complicate non-transactional filesystems.
>> >> Those filesystems should just flush cmtime at the end of writepages.
>> >
>> > do_writepages() is the wrong place to do such updates - we can get
>> > writeback directly through .writepage, so the time updates need to
>> > be in .writepage. That first .writepage call will clear the bit on
>> > the mapping, so it's only done on the first call to .writepage on
>> > the given mapping.
>>
>> Last time I checked, all the paths that actually needed the timestamp
>> update went through .writepages.  I'll double-check.
>   kswapd can call just .writepage to do the writeout so timestamp update
> should be handled there as well. Otherwise all pages in a mapping can be
> cleaned without timestamp being updated.

OK, I'll fix that.

>
> Which btw made me realize that even your scheme doesn't completely make
> sure timestamp is updated after mmap write - if you have pages 0 and 1, you
> write to both of them - CMTIME flag gets set. Then fsync_range(fd, 0, 4096)
> is called. We write the page 0, writeprotect it, update timestamps. But
> page 1 is still writeable so writes to it won't set CMTIME flag, neither
> update the timestamp... Not that I think this can be reasonably solved but
> it is a food for thought.

This should already work.  AS_CMTIME is set when the pte goes from
dirty to clean, not when the pte goes from wp to writable.  So
whenever clear_page_dirty_for_io is called on page 1, AS_CMTIME will
be set and a subsequent writepages call will update the timestamp.

--Andy
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