Re: [PATCH 2/2] mm: numa: Do not mark PTEs pte_numa when splitting huge pages

[Hugh Dickins]

On Thu, 2 Oct 2014, 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.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

except for where you say "it should get fixed up by change_pte_range()".
Well, I agree it "should", but it does not: because once the pte has
both _PAGE_NUMA and _PAGE_PROTNONE on it, then it fails our pte_numa()
test, and so _PAGE_NUMA is not cleared, even if later replacing
_PAGE_PROTNONE by _PAGE_PRESENT (whereupon the _PAGE_NUMA looks like
_PAGE_SPECIAL).

This patch is clearly safe, and fixes a real bug, almost certainly the
one seen by Sasha; but I still can't tie the ends together to see how
it would explain the endless refaulting seen by Dave.

> ---
>  mm/huge_memory.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d9a21d06..f8ffd94 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1795,14 +1795,17 @@ static int __split_huge_page_map(struct page *page,
>  		for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
>  			pte_t *pte, entry;
>  			BUG_ON(PageCompound(page+i));
> +			/*
> +			 * Note that pmd_numa is not transferred deliberately
> +			 * to avoid any possibility that pte_numa leaks to
> +			 * a PROT_NONE VMA by accident.
> +			 */
>  			entry = mk_pte(page + i, vma->vm_page_prot);
>  			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>  			if (!pmd_write(*pmd))
>  				entry = pte_wrprotect(entry);
>  			if (!pmd_young(*pmd))
>  				entry = pte_mkold(entry);
> -			if (pmd_numa(*pmd))
> -				entry = pte_mknuma(entry);
>  			pte = pte_offset_map(&_pmd, haddr);
>  			BUG_ON(!pte_none(*pte));
>  			set_pte_at(mm, haddr, pte, entry);
> -- 
> 1.8.4.5
> 
> 
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