Re: [bug report] mm, hwpoison: memory_failure races with alloc_fresh_huge_page/free_huge_page

[Naoya Horiguchi <naoya.horiguchi@linux.dev>]

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.

- 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.
> 
> Thanks,
> Naoya Horiguchi