Re: [PATCH] mm: hugetlb: fix a race between memory-failure/soft_offline and gather_surplus_pages

HORIGUCHI NAOYA(堀口 直也)
2021-04-23T08:02:17Z

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.

Thanks,
Naoya Horiguchi