Re: [PATCH v4 1/2] mm,hwpoison: fix race with compound page allocation

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Mon, May 17, 2021 at 01:11:25PM -0700, Mike Kravetz wrote:
> On 5/16/21 9:54 PM, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > 
> > When hugetlb page fault (under overcommitting 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 a time
> > window where compound pages have non-zero refcount during initialization.
> > 
> > So makes __get_hwpoison_page() check page status a bit more 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+
> > ---
> > ChangeLog v4:
> > - all hugetlb related check in hugetlb_lock,
> > - fix build error with #ifdef CONFIG_HUGETLB_PAGE
> > ---
> >  mm/memory-failure.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git v5.12/mm/memory-failure.c v5.12_patched/mm/memory-failure.c
> > index a3659619d293..761f982b6d7b 100644
> > --- v5.12/mm/memory-failure.c
> > +++ v5.12_patched/mm/memory-failure.c
> > @@ -1094,6 +1094,16 @@ static int page_action(struct page_state *ps, struct page *p,
> >  static int __get_hwpoison_page(struct page *page)
> >  {
> >  	struct page *head = compound_head(page);
> > +	int ret = 0;
> > +
> > +#ifdef CONFIG_HUGETLB_PAGE
> > +	spin_lock(&hugetlb_lock);
> > +	if (PageHuge(head) && (HPageFreed(head) || HPageMigratable(head)))
> > +		ret = get_page_unless_zero(head);
> > +	spin_unlock(&hugetlb_lock);
> > +	if (ret > 0)
> > +		return ret;
> > +#endif
> 
> I must be missing something.
> 
> The above code makes sure the page is not in one of these transitive
> hugetlb states as mentioned in the commit message.  It only attempts
> to take a reference on the page if it is not in one of these states.
> 
> However, if it is in such a transitive state (!HPageFreed(head) &&
> !HPageMigratable(head)) we will fall through and execute the code:
> 
> 	if (get_page_unless_zero(head)) {
> 		if (head == compound_head(page))
> 			return 1;
> 
> So, it seems like we will always do a get_page_unless_zero() for
> PageHuge() pages?

Right, no need to fall through in such a case.

> 
> Also, due to commit db71ef79b59b ("hugetlb: make free_huge_page irq
> safe") you need to make sure interrupts are disabled when taking
> hugetlb_lock.

Thanks, I'll rebase to latest mainline and comply with new semantics.

- Naoya