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From:   Yang Shi <shy828301@gmail.com>
Date:   Mon, 1 Nov 2021 13:11:33 -0700
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Subject: Re: [v5 PATCH 6/6] mm: hwpoison: handle non-anonymous THP correctly
To:     Jue Wang <juew@google.com>
Cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
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        <naoya.horiguchi@nec.com>, Oscar Salvador <osalvador@suse.de>,
        Peter Xu <peterx@redhat.com>,
        Matthew Wilcox <willy@infradead.org>
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On Mon, Nov 1, 2021 at 12:38 PM Jue Wang <juew@google.com> wrote:
>
> A related bug but whose fix may belong to a separate series:
>
> split_huge_page fails when invoked concurrently on the same THP page.
>
> It's possible that multiple memory errors on the same THP get consumed
> by multiple threads and come down to split_huge_page path easily.

Yeah, I think it should be a known problem since the very beginning.
The THP split requires to pin the page and does check if the refcount
is expected or not and freezes the refcount if it is expected. So if
two concurrent paths try to split the same THP, one will fail due to
the pin from the other path, but the other one will succeed.

I don't think of a better way to remediate it other than retrying from
the very start off the top of my head. We can't simply check if it is
still a THP or not since THP split will just move the refcount pin to
the poisoned subpage so the retry path will lose the refcount for its
poisoned subpage.

Did you run into this problem on any real production environment? Or
it is just a artificial test case? I'm wondering if the extra
complexity is worth or not.

>
> Thanks,
> -Jue