linux-ext4 - Re: [PATCH v2 2/3] jbd2: Add extra parameter in start_this

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Date:	Sun, 5 Jun 2011 23:21:13 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Manish Katiyar <mkatiyar@...il.com>, linux-ext4@...r.kernel.org,
	mfasheh@...e.com, jlbec@...lplan.org
Subject: Re: [PATCH v2 2/3] jbd2: Add extra parameter in
 start_this_handle() to control allocation flags.

On Thu, Jun 02, 2011 at 11:54:24AM +0200, Jan Kara wrote:
>   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...

We don't actully call set_page_writeback() until right before we
submit the page for writeback.  And we convert the unwritten extents
in a workqueue, which gets submitted after we call
end_page_writeback().  So I'm still not seeing a problem; sorry if I'm
being dense!

			    	      	  - Ted

> 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.
> 
> 								Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
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