linux-kernel - Re: [PATCH 5/9] writeback: introduce the pageout work

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Message-Id: <20120306163742.b71bf57b.akpm@linux-foundation.org>
Date:	Tue, 6 Mar 2012 16:37:42 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Greg Thelen <gthelen@...gle.com>,
	Ying Han <yinghan@...gle.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/9] writeback: introduce the pageout work

On Sat, 3 Mar 2012 21:25:55 +0800
Fengguang Wu <fengguang.wu@...el.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.

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