Re: Latency writing to an mlocked ext4 mapping

[Andy Lutomirski]

On Wed, Oct 19, 2011 at 7:17 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Oct 19, 2011 at 6:15 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Wed, Oct 19, 2011 at 6:02 PM, Andreas Dilger <adilger@xxxxxxxxx> wrote:
>>> What kernel are you using?  A change to keep pages consistent during writeout was landed not too long ago (maybe Linux 3.0) in order to allow checksumming of the data.
>>
>> 3.0.6, with no relevant patches.  (I have a one-liner added to the tcp
>> code that I'll submit sometime soon.)  Would this explain the latency
>> in file_update_time or is that a separate issue?  file_update_time
>> seems like a good thing to make fully asynchronous (especially if the
>> file in question is a fifo, but I've already moved my fifos to tmpfs).
>
> On 2.6.39.4, I got one instance of:
>
> call_rwsem_down_read_failed ext4_map_blocks ext4_da_get_block_prep
> __block_write_begin ext4_da_write_begin ext4_page_mkwrite do_wp_page
> handle_pte_fault handle_mm_fault do_page_fault page_fault
>
> but I'm not seeing the large numbers of the ext4_page_mkwrite trace
> that I get on 3.0.6.  file_update_time is now by far the dominant
> cause of latency.

The culprit seems to be do_wp_page -> file_update_time ->
mark_inode_dirty_sync.  This surprises me for two reasons:

 - Why the _sync?  Are we worried that data will be written out before
the metadata?  If so, surely there's a better way than adding latency
here.

 - Why are we calling file_update_time at all?  Presumably we also
update the time when the page is written back (if not, that sounds
like a bug, since the contents may be changed after something saw the
mtime update), and, if so, why bother updating it on the first write?
Anything that relies on this behavior is, I think, unreliable, because
the page could be made writable arbitrarily early by another program
that changes nothing.

--Andy
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