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

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Message-ID: <CALCETrXqZ3Fgp9JHRJZCRfW-+qq2T_xw7Y_xyn2i=xK5+pdZoQ@mail.gmail.com>
Date:	Tue, 20 Aug 2013 12:27:31 -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 9:42 AM, Andy Lutomirski <luto@...capital.net> wrote:
> 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.

This is a bit ugly.  mpage_writepages and generic_writepages both call
->writepage.  If writepage starts checking cmtime, then writepages
will do multiple timestamp updates on filesystems that use this code.

I can modify vmscan and migrate to flush cmtime -- they seem to be the
only callers of ->writepage that aren't themselves called from
->writepages.

--Andy
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