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

HORIGUCHI NAOYA(堀口 直也)
2021-04-22T08:27:52Z

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
}

Thanks,
Naoya Horiguchi