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SubjectRe: [PATCH] mm: thp: fix potential clearing to referenced flag in page_idle_clear_pte_refs_one()
On Tue,  6 Feb 2018 08:06:36 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:

> For PTE-mapped THP, the compound THP has not been split to normal 4K
> pages yet, the whole THP is considered referenced if any one of sub
> page is referenced.
>
> When walking PTE-mapped THP by pvmw, all relevant PTEs will be checked
> to retrieve referenced bit. But, the current code just returns the
> result of the last PTE. If the last PTE has not referenced, the
> referenced flag will be cleared.
>
> So, here just break pvmw walk once referenced PTE is found if the page
> is a part of THP.
>
> ...
>
> --- a/mm/page_idle.c
> +++ b/mm/page_idle.c
> @@ -67,6 +67,14 @@ static bool page_idle_clear_pte_refs_one(struct page *page,
> if (pvmw.pte) {
> referenced = ptep_clear_young_notify(vma, addr,
> pvmw.pte);
> + /*
> + * For PTE-mapped THP, one sub page is referenced,
> + * the whole THP is referenced.
> + */
> + if (referenced && PageTransCompound(pvmw.page)) {
> + page_vma_mapped_walk_done(&pvmw);
> + break;
> + }

This means that the function will no longer clear the referenced bits
in all the ptes. What effect does this have and should we document
this in some fashion?

> } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> referenced = pmdp_clear_young_notify(vma, addr,
> pvmw.pmd);

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