Re: [PATCH v2 2/3] jbd2: Add extra parameter in start_this_handle() to control allocation flags.

[Manish Katiyar <mkatiyar@gmail.com>]

On Thu, Jun 2, 2011 at 2:54 AM, Jan Kara <jack@suse.cz> wrote:
> On Tue 31-05-11 18:27:20, Ted Tso wrote:
>> On Tue, May 31, 2011 at 01:22:53PM +0200, Jan Kara wrote:
>> >
>> > The problem is that with ext4, we need i_mutex in io completion path to
>> > end page writeback. So we cannot do GFP_KERNEL allocation whenever we hold
>> > i_mutex because mm might wait in direct reclaim for IO to complete and that
>> > cannot happen until we release i_mutex.
>>
>> OK, maybe I'm being dense, but I'm not seeing it.  I see where we need
>> i_mutex on the ext4_da_writepages() codepath, but that's never used
>> for direct reclaim.  Direct reclaim only calls ext4_writepage(), and
>> that doesn't seem to try to grab i_mutex as near as I can tell.  Am I
>> missing something?
>  What happens is that direct reclaim sometimes does
> wait_on_page_writeback() (e.g. shrink_page_list()) or it explicitely waits
> for NR_WRITEBACK statistics to go below some threshold
> (throttle_vm_writeout()). And that is deadlockable if we hold i_mutex while
> doing this because we may need i_mutex to actually move the page from
> PageWriteback state...
>
> As I'm saying this, I've realized ext4 has this problem also with
> stable-pages patches because there we can wait for PageWriteback in
> grab_cache_page_write_begin() when we also hold i_mutex. So I think we'll
> have to come up with a way to convert unwritten extents without having to
> hold i_mutex. That's going to be interesting.

Hi Jan/Ted,

Does that mean I should remove the whole JBD2_TOPLEVEL thing from my
revised patch ? Or should I fix it as per your feedback in
the other patch ?

-- 
Thanks -
Manish
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