Re: [PATCH V2] mm: mempolicy: skip non-migratable VMAs when setting MPOL_MF_LAZY

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

Hello Liang,

On Thu, Jan 07, 2016 at 11:52:38AM +0800, Liang Chen wrote:
> MPOL_MF_LAZY is not visible from userspace since 'commit a720094ded8c
> ("mm: mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now")'
> , but it should still skip non-migratable VMAs such as VM_IO, VM_PFNMAP,
> and VM_HUGETLB VMAs, and avoid useless overhead of minor faults.
> 
> Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
> Signed-off-by: Gavin Guo <gavin.guo@canonical.com>
> ---
> Changes since v2:
> - Add more description into the changelog
> 
> We have been evaluating the enablement of MPOL_MF_LAZY again, and found
> this issue. And we decided to push this patch upstream no matter if we
> finally determine to propose re-enablement of MPOL_MF_LAZY or not. Since
> it can be a potential problem even if MPOL_MF_LAZY is not enabled this
> time.
> ---
>  mm/mempolicy.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 87a1779..436ff411 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -610,7 +610,8 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
>  
>  	if (flags & MPOL_MF_LAZY) {
>  		/* Similar to task_numa_work, skip inaccessible VMAs */
> -		if (vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
> +		if (vma_migratable(vma) &&
> +			vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
>  			change_prot_numa(vma, start, endvma);
>  		return 1;
>  	}

task_numa_work() does more vma checks before entering change_prot_numa() like
vma_policy_mof(), is_vm_hugetlb_page(), and (vma->vm_flags & VM_MIXEDMAP).
So is it better to use the same check set to limit the target vmas to auto-numa
enabled ones?

Thanks,
Naoya Horiguchi
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