linux-kernel - Re: [RFC][PATCH] mm: couple rcu and memory reclaim

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Date:	Mon, 24 Sep 2007 14:58:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	balbir@...ux.vnet.ibm.com
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH] mm: couple rcu and memory reclaim

On Mon, 24 Sep 2007 18:18:30 +0530 Balbir Singh
<balbir@...ux.vnet.ibm.com> wrote:

> Peter Zijlstra wrote:
> >>> Only kswapd can do this, direct reclaim has deadlock potential.
> >> Yes, but not in all cases, do you want to add any gfp_mask
> >> based smartness for direct reclaim?
> > 
> > gfp_mask doesn't carry the needed information. It depends on whether
> > the current context holds a rcu_read_lock().
> > 
> 
> What I meant was that nobody would hold rcu_read_lock() and pass
> gfp_mask of GFP_KERNEL in scan_control or to do_try_to_free_pages()
> 
> > so something like:
> > 
> >    rcu_read_lock()
> >    foo = kmalloc(sizeof(foo))
> 
> At this point, you really can't use GFP_KERNEL, since rcu_read_lock()
> disables pre-emption in the current kernel, ideally you should see
> a might_sleep() BUG.
> 
> >       new_slab()
> >         __alloc_pages()
> >           try_to_free_pages()
> >             synchronise_rcu() <-- deadlock
> >    rcu_read_unlock()

I guess I've been using preemptibe rcu for way too long. Would need to
clean up some of -rt to make this true there, but should be doable
indeed.

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