On 2022/8/4 15:44, Naoya Horiguchi wrote: > On Thu, Aug 04, 2022 at 04:40:25PM +0900, Naoya Horiguchi wrote: >> On Tue, Aug 02, 2022 at 02:27:36PM +0800, Miaohe Lin wrote: >>> On 2022/8/2 12:07, Muchun Song wrote: >>>> On Tue, Aug 02, 2022 at 10:00:50AM +0800, Miaohe Lin wrote: >>>>> Hi all: >>>>> When I investigate the mm/memory-failure.c code again, I found there's a possible race window >>>>> between memory_failure and alloc_fresh_huge_page/free_huge_page. Thank about the below scene: >>>>> >>>>> CPU 1 CPU 2 >>>>> alloc_fresh_huge_page -- page refcnt > 0 memory_failure >>>>> prep_new_huge_page get_huge_page_for_hwpoison >>>>> !PageHeadHuge -- so 2(not a hugepage) is returned >>>>> hugetlb_vmemmap_optimize -- subpages is read-only >>>>> set_compound_page_dtor -- PageHuge is true now, but too late!!! >>>>> TestSetPageHWPoison(p) >>>>> -- We might write to read-only subpages here!!! >>>>> >>>>> Another similar scene: >>>>> >>>>> CPU 1 CPU 2 >>>>> free_huge_page -- page refcnt == 0 and not PageHuge memory_failure >>>>> get_huge_page_for_hwpoison >>>>> !PageHeadHuge -- so 2(not a hugepage) is returned >>>>> TestSetPageHWPoison(p) >>>>> -- We might write to read-only subpages here!!! >>>>> hugetlb_vmemmap_restore -- subpages can be written to now, but too late!!! >>>>> >>>> >>>> I agree this race is possible, I have proposed this race in thread [1]. >> >> Thank you for reminding this, and I agree that we need some solution. >> >>> >>> Oh, I remember I see the race proposed in [1] but I did not look into that carefully at that time. Sorry. >>> >>>> But I didn't think more how to solve it. >>> I hope this thread can find a good solution. :) >> >> Both of the races show that __get_huge_page_for_hwpoison() fails to >> capture the case of generic compound page during turning into hugetlb, > > I noticed my wrong description just after sending this out. The second race > is about generic compound page turning into "buddy", sorry. Never mind. Thanks for your thought. > > - Naoya Horiguchi > >> What makes things complicated is that no one knows to which state >> such a compound page finally turns into. So I think that if the page >> to be handled is an unknown compound page, we need to wait until it >> becomes some known page state to avoid misjudging. >> >> If we need a quick small fix, we may replace the check "!PageHeadHuge()" >> in __get_huge_page_for_hwpoison() with "!PageCompound()", and add another >> retry path in get_huge_page_for_hwpoison() for non-hugetlb compound pages. Sounds like a good idea. If we can figure out a stable way to tell it from other compound page, it will be worth fixing it. Let's have a try. :) >> >> Thanks, >> Naoya Horiguchi Many thanks for your suggestion, Naoya. > . >
