From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [patch 10/21] buffer heads: Support slab defrag
Date: Tue, 20 May 2008 16:28:16 -0700
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Cc: Evgeniy Polyakov <johnpol@2ka.mipt.ru>,
	Christoph Lameter <clameter@sgi.com>,
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To: David Chinner <dgc@sgi.com>
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On Wed, 21 May 2008 09:19:42 +1000 David Chinner <dgc@sgi.com> wrote:

> sync_inode() is far too
> heavy-weight to be used in a reclaim context

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.