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

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Date:	Mon, 24 Sep 2007 18:18:30 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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

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()


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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