Re: [PATCH v3 3/5] mm: Notify filesystems when it's time to apply adeferred cmtime update

[Andy Lutomirski]

On Tue, Aug 20, 2013 at 2:48 PM, Dave Chinner <david@xxxxxxxxxxxxx> 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@xxxxxxx> 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?

>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx



-- 
Andy Lutomirski
AMA Capital Management, LLC
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