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

[Naoya Horiguchi]

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!
> >>
> > .
> > 
>