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SubjectRe: [bug report] mm, hwpoison: memory_failure races with alloc_fresh_huge_page/free_huge_page
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].
But I didn't think more how to solve it.

[1] https://lore.kernel.org/linux-mm/20220623235153.2623702-1-naoya.horiguchi@linux.dev/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.
Just a quick reply, no suggestion/solutions from me.

Thanks.

> Any response would be appreciated!
>
> Thanks!
>

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