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    SubjectRe: [PATCH] mm: thp: fix potential clearing to referenced flag in page_idle_clear_pte_refs_one()
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    On 2/9/18 12:16 AM, Kirill A. Shutemov wrote:
    > On Thu, Feb 08, 2018 at 08:47:35PM -0800, Yang Shi wrote:
    >>
    >> On 2/8/18 8:33 PM, Kirill A. Shutemov wrote:
    >>> On Thu, Feb 08, 2018 at 02:39:26PM -0800, Andrew Morton wrote:
    >>>> 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?
    >>> Yeah, the patch is wrong. We need to get all ptes for THP cleared.
    >>>
    >>> What about something like this instead (untested):
    >> Thanks, Kirill. It looks correct. All ptes should be cleared.
    >>
    >> I'm going to prepare v2 patch.
    > Note, it should be ||=, not |= (although it would work correctly too).

    checkpatch doesn't like "||=", it reports no space error. Use referenced
    = referenced || ptep_clear_young_notify(...) to make checkpatch happy.

    Regards,
    Yang

    >
    > I should really wake up properly before touching code. :-/
    >

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