linux-kernel - 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 17:47:10 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>,
	"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 3:43 PM, Dave Chinner <david@...morbit.com> wrote:
> On Tue, Aug 20, 2013 at 02:54:01PM -0700, Andy Lutomirski wrote:
>> On Tue, Aug 20, 2013 at 2:48 PM, Dave Chinner <david@...morbit.com> wrote:
>> > On Tue, Aug 20, 2013 at 09:42:34AM -0700, Andy Lutomirski 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.
>> >>
>> >> >
>> >> > 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.
>> >
>> > Oh, I missed that - I thought you were setting AS_CMTIME during
>> > .page_mkwrite.
>> >
>> > Setting it in clear_page_dirty_for_io() is too late for filesystems
>> > to include it in their existing transactions during .writepage, (at
>> > least for XFs and ext4) because they do their delayed allocation
>> > transactions before changing page state....
>>
>> Couldn't it go between mpage_map_and_submit_extent and
>> ext4_journal_stop in ext4_writepages?
>
> Maybe - I'm not an ext4 expert - but even if you can make it work
> for ext4 in some way, that doesn't mean it is possible for any other
> filesystem to use the same method. You're adding code to generic,
> non-filesystem specific code paths and so the solutions need to be
> generic rather not tied to how a specific filesystem is implemented.
>

I don't see the problem for xfs or btrfs either.

xfs uses generic_writepages, which already does the right thing.  (xfs
with my updated patches passes my tests.)  xfs_vm_writepage calls
xfs_start_page_writeback(..., 1, ...), so clear_page_dirty_for_io is
called.  At that point (I presume), it would still be possible to add
metadata to a transaction (assuming there's a transaction open -- I
don't have a clue here).  Even if this is too late, xfs_vm_writepage
could call page_mkwrite to for AS_CMTIME to be set if needed.
page_mkwrite will be fast if the page isn't mmapped.  What am I
missing?

btrfs seems to do much the same thing.

--Andy


> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@...morbit.com



-- 
Andy Lutomirski
AMA Capital Management, LLC
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