Re: [PATCH] mm: thp: fix potential clearing to referenced flag in page_idle_clear_pte_refs_one()

[Andrew Morton]

On Tue,  6 Feb 2018 08:06:36 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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);