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

HORIGUCHI NAOYA(堀口 直也)
2021-05-07T04:17:59Z

On Thu, May 06, 2021 at 10:51:33AM +0200, Oscar Salvador wrote:
> 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.

Thanks, we had better add rechecking as we do in dissolve_free_huge_page().

> 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)?

Yes, retry could work in this case. Looking at existing code,
get_any_page() provides "retry" layer, but it's not called now by
get_hwpoison_page() when called from memory_failure(). So I think of trying
to adjust code and make get_hwpoison_page call get_any_page() instead of
calling __get_hwpoison_page(() directly.

Thanks,
Naoya Horiguchi