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Date: Tue, 6 Mar 2012 16:37:42 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Fengguang Wu <fengguang.wu@intel.com>
Cc: Jan Kara <jack@suse.cz>, Greg Thelen <gthelen@google.com>,
        Ying Han <yinghan@google.com>,
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        Minchan Kim <minchan.kim@gmail.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/9] writeback: introduce the pageout work
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On Sat, 3 Mar 2012 21:25:55 +0800
Fengguang Wu <fengguang.wu@intel.com> wrote:

> > > get_page() looks the perfect solution to verify if the struct inode
> > > pointer (w/o igrab) is still live and valid.
> > > 
> > > [...upon rethinking...] Oh but still we need to lock some page to pin
> > > the inode during the writeout. Then there is the dilemma: if the page
> > > is locked, we effectively keep it from being written out...
> > 
> > No, all you need to do is to structure the code so that after the page
> > gets unlocked, the kernel thread does not touch the address_space.  So
> > the processing within the kthread is along the lines of
> > 
> > writearound(locked_page)
> > {
> > 	write some pages preceding locked_page;	/* touches address_space */
> 
> It seems the above line will lead to ABBA deadlock.
> 
> At least btrfs will lock a number of pages in lock_delalloc_pages().

Well, this code locks multiple pages too.  I forget what I did about
that - probably trylock.  Dirty pages aren't locked for very long.