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 18:24:36 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	riel@...hat.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 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.
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