linux-kernel - Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all

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Message-ID: <1270138354.1598.99.camel@laptop>
Date:	Thu, 01 Apr 2010 18:12:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	Kent Overstreet <kent.overstreet@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in
 mmu_take_all_locks()

On Thu, 2010-04-01 at 17:42 +0200, Andrea Arcangeli wrote:
> The ugliest part of it (that I think you missed below) is the breakage
> of the RCU locking in the anon-vma which requires adding refcounting
> to it. That was the worst part of the conversion as far as I can tell.
> 
One thing we can do there is to mutex_trylock() if we get the lock, see
if we've got the right object, if the trylock fails we can do the
refcount thing and sleep. That would allow the fast-path to remain a
single atomic.

The only thing is growing that anon_vma struct, but KSM seems to already
do that for the most common .configs.



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