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

[Muchun Song]

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@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.
Just a quick reply, no suggestion/solutions from me.

Thanks.

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