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, 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. Thanks, Naoya Horiguchi > > > > > [1] https://lore.kernel.org/linux-mm/20220623235153.2623702-1-naoya.horiguchi@xxxxxxxxx/T/#ma094a7cea7df8fd9a77a91551bf39077d89e23bd > > > >> I think the above scenes are possible. But I can't found a stable solution to fix it. Any suggestions? > >> Or is it not worth to fix it as it's too rare? Or am I miss something? > >> > > > > Luckily, the system will panic at once, which encountering this race. However, > > we don't see any bug report. If we have an easy way to fix it, I think it is worth. > > Agree. But I can't find a easy way to fix it yet. > > > Just a quick reply, no suggestion/solutions from me. > > Many thanks for your quick reply. > > > > > Thanks. > > > >> Any response would be appreciated! > >> > >> Thanks! > >> > > . > > >
