Re: [PATCH 4/4] mm: numa: Do not mark PTEs pte_numa when splitting huge pages

[Kirill A. Shutemov]

On Thu, Oct 02, 2014 at 02:29:18PM +0100, Mel Gorman wrote:
> This patch reverts 1ba6e0b50b ("mm: numa: split_huge_page: transfer the
> NUMA type from the pmd to the pte"). If a huge page is being split due
> a protection change and the tail will be in a PROT_NONE vma then NUMA
> hinting PTEs are temporarily created in the protected VMA.
> 
>  VM_RW|VM_PROTNONE
> |-----------------|
>       ^
>       split here
> 
> In the specific case above, it should get fixed up by change_pte_range()
> but there is a window of opportunity for weirdness to happen. Similarly,
> if a huge page is shrunk and split during a protection update but before
> pmd_numa is cleared then a pte_numa can be left behind.
> 
> Instead of adding complexity trying to deal with the case, this patch
> will not mark PTEs NUMA when splitting a huge page. NUMA hinting faults
> will not be triggered which is marginal in comparison to the complexity
> in dealing with the corner cases during THP split.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

I think it deserves a comment to avoid re-introducing NUMA type
transferring.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov
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