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

Naoya Horiguchi
2021-04-28T07:47:01Z

On Fri, Apr 23, 2021 at 08:01:54AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Thu, Apr 22, 2021 at 08:27:46AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Wed, Apr 21, 2021 at 11:03:24AM -0700, Mike Kravetz wrote:
> > > On 4/21/21 1:33 AM, HORIGUCHI NAOYA(堀口 直也) wrote:
> > > > On Wed, Apr 21, 2021 at 10:03:34AM +0200, Michal Hocko wrote:
> > > >> [Cc Naoya]
> > > >>
> > > >> On Wed 21-04-21 14:02:59, Muchun Song wrote:
> > > >>> The possible bad scenario:
> > > >>>
> > > >>> 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)
> > > >>>
> > > >>> The refcount can possibly be increased by memory-failure or soft_offline
> > > >>> handlers, we can trigger VM_BUG_ON_PAGE and wrongly add the page to the
> > > >>> hugetlb pool list.
> > > >>
> > > >> The hwpoison side of this looks really suspicious to me. It shouldn't
> > > >> really touch the reference count of hugetlb pages without being very
> > > >> careful (and having hugetlb_lock held).
> > > >
> > > > I have the same feeling, there is a window where a hugepage is refcounted
> > > > during converting from buddy free pages into free hugepage, so refcount
> > > > alone is not enough to prevent the race. hugetlb_lock is retaken after
> > > > alloc_surplus_huge_page returns, so simply holding hugetlb_lock in
> > > > get_hwpoison_page() seems not work. Is there any status bit to show that a
> > > > hugepage is just being initialized (not in free hugepage pool or in use)?
> > > >
> > >
> > > It seems we can also race with the code that makes a compound page a
> > > hugetlb page. The memory failure code could be called after allocating
> > > pages from buddy and before setting compound page DTOR. So, the memory
> > > handling code will process it as a compound page.
> >
> > Yes, so get_hwpoison_page() has to call get_page_unless_zero()
> > only when memory_failure() can surely handle the error.
> >
> > >
> > > Just thinking that this may not be limited to the hugetlb specific memory
> > > failure handling?
> >
> > Currently hugetlb page is the only type of compound page supported by memory
> > failure. But I agree with you that other types of compound pages have the
> > same race window, and judging only with get_page_unless_zero() is dangerous.
> > So I think that __get_hwpoison_page() should have the following structure:
> >
> > if (PageCompound) {
> > if (PageHuge) {
> > if (PageHugeFreed || PageHugeActive) {
> > if (get_page_unless_zero)
> > return 0; // path for in-use hugetlb page
> > else
> > return 1; // path for free hugetlb page
> > } else {
> > return -EBUSY; // any transient hugetlb page
> > }
> > } else {
> > ... // any other compound page (like thp, slab, ...)
> > }
> > } else {
> > ... // any non-compound page
> > }
>
> The above pseudo code was wrong, so let me update my thought.
> I'm now trying to solve the reported issue by changing __get_hwpoison_page()
> like below:
>
> static int __get_hwpoison_page(struct page *page)
> {
> struct page *head = compound_head(page);
>
> if (PageCompound(page)) {
> if (PageSlab(page)) {
> return get_page_unless_zero(page);
> } else if (PageHuge(head)) {
> if (HPageFreed(head) || HPageMigratable(head))
> return get_page_unless_zero(head);
> } else if (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 (get_page_unless_zero(head)) {
> if (head == compound_head(page))
> return 1;
> pr_info("Memory failure: %#lx cannot catch tail\n",
> page_to_pfn(page));
> put_page(head);
> }
> }
> return 0;
> }
>
> return get_page_unless_zero(page);
> }
>
> Some notes:
>
> - in hugetlb path, new HPage* checks should avoid the reported race,
> but I still need more testing to confirm it,
> - PageSlab check is added because otherwise I found that "non anonymous thp"
> path is chosen, that's obviously wrong,
> - thp's branch has a known issue unrelated to the current issue, which
> will/should be improved later.
>
> I'll send a patch next week.

I confirmed that the patch fixes the reported problem (in the testcase
triggering VM_BUG_ON_PAGE() without this patch).
So let me suggest this as a fix on hwpoison side.

Thanks,
Naoya Horiguchi

---
From: Naoya Horiguchi <naoya.horiguchi@nec.com>
Date: Wed, 28 Apr 2021 15:55:47 +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 for slab pages.

Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Reported-by: Muchun Song <songmuchun@bytedance.com>
---
mm/memory-failure.c | 48 +++++++++++++++++++++++++--------------------
1 file changed, 27 insertions(+), 21 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index a3659619d293..61988e332712 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1095,30 +1095,36 @@ 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)) {
+ if (HPageFreed(head) || HPageMigratable(head))
+ return get_page_unless_zero(head);
+ } else if (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 (get_page_unless_zero(head)) {
+ if (head == compound_head(page))
+ return 1;
+ pr_info("Memory failure: %#lx cannot catch tail\n",
+ page_to_pfn(page));
+ put_page(head);
+ }
}
+ return 0;
}

- if (get_page_unless_zero(head)) {
- if (head == compound_head(page))
- return 1;
-
- pr_info("Memory failure: %#lx cannot catch tail\n",
- page_to_pfn(page));
- put_page(head);
- }
-
- return 0;
+ return get_page_unless_zero(page);
}

/*
--
2.25.1