Re: [PATCH v2] mm,hwpoison: fix race with compound page allocation

Oscar Salvador
2021-05-06T08:51:40Z

On Thu, May 06, 2021 at 10:31:22AM +0900, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> Date: Thu, 6 May 2021 09:54:39 +0900
> Subject: [PATCH] mm,hwpoison: fix race with compound page allocation
>
> When hugetlb page fault (under overcommiting situation) and memory_failure()
> race, VM_BUG_ON_PAGE() is triggered by the following race:
>
> CPU0: CPU1:
>
> gather_surplus_pages()
> page = alloc_surplus_huge_page()
> memory_failure_hugetlb()
> get_hwpoison_page(page)
> __get_hwpoison_page(page)
> get_page_unless_zero(page)
> zero = put_page_testzero(page)
> VM_BUG_ON_PAGE(!zero, page)
> enqueue_huge_page(h, page)
> put_page(page)
>
> __get_hwpoison_page() only checks page refcount before taking additional
> one for memory error handling, which is wrong because there's time
> windows where compound pages have non-zero refcount during initialization.
>
> So makes __get_hwpoison_page() check more page status for a few types
> of compound pages. PageSlab() check is added because otherwise
> "non anonymous thp" path is wrongly chosen.
>
> Fixes: ead07f6a867b ("mm/memory-failure: introduce get_hwpoison_page() for consistent refcount handling")
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> Reported-by: Muchun Song <songmuchun@bytedance.com>
> Cc: stable@vger.kernel.org # 5.12+

Hi Naoya,

thanks for the patch.
I have some concerns though, more below:

> ---
> mm/memory-failure.c | 53 +++++++++++++++++++++++++++------------------
> 1 file changed, 32 insertions(+), 21 deletions(-)
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index a3659619d293..966a1d6b0bc8 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1095,30 +1095,41 @@ static int __get_hwpoison_page(struct page *page)
> {
> struct page *head = compound_head(page);
>
> - if (!PageHuge(head) && PageTransHuge(head)) {
> - /*
> - * Non anonymous thp exists only in allocation/free time. We
> - * can't handle such a case correctly, so let's give it up.
> - * This should be better than triggering BUG_ON when kernel
> - * tries to touch the "partially handled" page.
> - */
> - if (!PageAnon(head)) {
> - pr_err("Memory failure: %#lx: non anonymous thp\n",
> - page_to_pfn(page));
> - return 0;
> + if (PageCompound(page)) {
> + if (PageSlab(page)) {
> + return get_page_unless_zero(page);
> + } else if (PageHuge(head)) {
> + int ret = 0;
> +
> + spin_lock(&hugetlb_lock);
> + if (HPageFreed(head) || HPageMigratable(head))
> + ret = get_page_unless_zero(head);
> + spin_unlock(&hugetlb_lock);
> + return ret;

Ok, I am probably overthinking this but should we re-check under the
lock wehther the page is a hugetlb page?
My concern is, what would happen if:

CPU0 CPU1
__get_hwpoison_page
PageHuge(head) == T
dissolve hugetlb page
hugetlb_lock

In that case, by the time we get to check hugetlb flags, those checks
might return false, and we do not get a refcount.
So, I guess my question is: Should we re-check under the lock, and if it
is not, do a "goto try_to_get_ref" that starts right at the beginning,
or goes directly to the get_page_unless_zero at the end (the former
probably better)?

As I said, I might be overthinking this, but well.

--
Oscar Salvador
SUSE L3