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

From: Andrew Morton
Date: Tue May 20 2008 - 19:45:17 EST

On Wed, 21 May 2008 09:19:42 +1000 David Chinner <dgc@xxxxxxx> 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.

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