linux-kernel - Re: [patch 10/21] buffer heads: Support slab defrag

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Message-ID: <20080521061531.GA27362@2ka.mipt.ru>
Date:	Wed, 21 May 2008 10:15:32 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Chinner <dgc@....com>, Christoph Lameter <clameter@....com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Mel Gorman <mel@...net.ie>, andi@...stfloor.org,
	Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>, mpm@...enic.com
Subject: Re: [patch 10/21] buffer heads: Support slab defrag

On Tue, May 20, 2008 at 04:28:16PM -0700, Andrew Morton (akpm@...ux-foundation.org) wrote:
> It's more than efficiency.  There are lots and lots of things we cannot
> do in direct-reclaim context.
> 
> a) Can't lock pages (well we kinda sorta could, but generally code
>    will just trylock)
> 
> b) Cannot rely on the inode or the address_space being present in
>    memory after we have unlocked the page.
> 
> c) Cannot run iput().  Or at least, we couldn't five or six years
>    ago.  afaik nobody has investigated whether the situation is now
>    better or worse.
> 
> d) lots of deadlock scenarios - need to test __GFP_FS basically everywhere
>    in which you share code with normal writeback paths.
> 
> Plus e), f), g) and h).  Direct-reclaim is a hostile environment. 
> Things like b) are a real killer - nasty, subtle, rare,
> memory-pressure-dependent crashes.

Which basically means we can not do direct writeback at reclaim time?..

-- 
	Evgeniy Polyakov
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