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

[David Rientjes]

On Tue, 18 Feb 2014, riel@xxxxxxxxxx wrote:

> From: Rik van Riel <riel@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Reported-by: Xing Gang <gang.xing@xxxxxx>
> Tested-by: Chegu Vinod <chegu_vinod@xxxxxx>

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

Might have been cleaner to move the 
mmu_notifier_invalidate_range_{start,end}() to hugetlb_change_protection() 
as well, though.
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