Messages in this thread | | | Date | Fri, 9 Feb 2018 11:16:38 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH] mm: thp: fix potential clearing to referenced flag in page_idle_clear_pte_refs_one() |
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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).
I should really wake up properly before touching code. :-/
-- Kirill A. Shutemov
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