linux-kernel - Re: [PATCH -mm 3/3] move mmu notifier call from change_protection to change_pmd

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Date:	Tue, 18 Feb 2014 22:08:27 -0500
From:	Rik van Riel <riel@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-mm@...ck.org, peterz@...radead.org, chegu_vinod@...com,
	aarcange@...hat.com, akpm@...ux-foundation.org
Subject: Re: [PATCH -mm 3/3] move mmu notifier call from change_protection
 to change_pmd_range

On 02/18/2014 09:24 PM, David Rientjes wrote:
> On Tue, 18 Feb 2014, riel@...hat.com wrote:
> 
>> From: Rik van Riel <riel@...hat.com>
>>
>> The NUMA scanning code can end up iterating over many gigabytes
>> of unpopulated memory, especially in the case of a freshly started
>> KVM guest with lots of memory.
>>
>> This results in the mmu notifier code being called even when
>> there are no mapped pages in a virtual address range. The amount
>> of time wasted can be enough to trigger soft lockup warnings
>> with very large KVM guests.
>>
>> This patch moves the mmu notifier call to the pmd level, which
>> represents 1GB areas of memory on x86-64. Furthermore, the mmu
>> notifier code is only called from the address in the PMD where
>> present mappings are first encountered.
>>
>> The hugetlbfs code is left alone for now; hugetlb mappings are
>> not relocatable, and as such are left alone by the NUMA code,
>> and should never trigger this problem to begin with.
>>
>> Signed-off-by: Rik van Riel <riel@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>> Reported-by: Xing Gang <gang.xing@...com>
>> Tested-by: Chegu Vinod <chegu_vinod@...com>
> 
> Acked-by: David Rientjes <rientjes@...gle.com>
> 
> Might have been cleaner to move the 
> mmu_notifier_invalidate_range_{start,end}() to hugetlb_change_protection() 
> as well, though.

I can certainly do that if you want.  Just let me know
and I'll send a v2 of patch 3 :)

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