linux-kernel - Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

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Date:	Tue, 15 Jun 2010 21:13:37 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

On Tue, Jun 15, 2010 at 06:53:41AM -0400, Christoph Hellwig wrote:
> > +	/*
> > +	 * XXX: This is the Holy Hand Grenade of PotentiallyInvalidMapping. As
> > +	 * the page lock has been dropped by ->writepage, that mapping could
> > +	 * be anything
> > +	 */
> 
> Why is this an XXX comment?

It's just a pretty simple use-after-free. Maybe people forget it because
->writepage is an asynchronous API.


> > + *
> > + * XXX: Is there a problem with holding multiple page locks like this?
> 
> I think there is.  There's quite a few places that do hold multiple
> pages locked, but they always lock pages in increasing page->inxex order.
> Given that this locks basically in random order it could cause problems
> for those places.

There shouldn't be a problem _holding_ the locks, but there is a problem
waiting for multiple locks out of page->index order.

But there is a problem with holding the lock of a lot of pages while
calling ->writepage on them. So yeah, you can't do that.

Hmm, I should rediff that lockdep page_lock patch and get it merged.
(although I don't know if that can catch these all these problems easily)

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