Re: [PATCH v5 1/2] mm,hwpoison: fix race with hugetlb page allocation

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

On Wed, May 19, 2021 at 03:32:17PM -0700, Mike Kravetz wrote:
> On 5/18/21 4:12 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 the page refcount before taking an
> > 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 make __get_hwpoison_page() check page status a bit
> > more for hugetlb pages.
> > 
> > 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+
> > ---
> >  include/linux/hugetlb.h |  6 ++++++
> >  mm/hugetlb.c            | 15 +++++++++++++++
> >  mm/memory-failure.c     |  8 +++++++-
> >  3 files changed, 28 insertions(+), 1 deletion(-)
> > 
> > diff --git v5.13-rc2/include/linux/hugetlb.h v5.13-rc2_patched/include/linux/hugetlb.h
> > index b92f25ccef58..790ae618548d 100644
> > --- v5.13-rc2/include/linux/hugetlb.h
> > +++ v5.13-rc2_patched/include/linux/hugetlb.h
> > @@ -149,6 +149,7 @@ bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
> >  long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
> >  						long freed);
> >  bool isolate_huge_page(struct page *page, struct list_head *list);
> > +int get_hwpoison_huge_page(struct page *page, bool *hugetlb);
> >  void putback_active_hugepage(struct page *page);
> >  void move_hugetlb_state(struct page *oldpage, struct page *newpage, int reason);
> >  void free_huge_page(struct page *page);
> > @@ -339,6 +340,11 @@ static inline bool isolate_huge_page(struct page *page, struct list_head *list)
> >  	return false;
> >  }
> >  
> > +static inline int get_hwpoison_huge_page(struct page *page, bool *hugetlb)
> > +{
> > +	return 0;
> > +}
> > +
> >  static inline void putback_active_hugepage(struct page *page)
> >  {
> >  }
> > diff --git v5.13-rc2/mm/hugetlb.c v5.13-rc2_patched/mm/hugetlb.c
> > index 95918f410c0f..f138bae3e302 100644
> > --- v5.13-rc2/mm/hugetlb.c
> > +++ v5.13-rc2_patched/mm/hugetlb.c
> > @@ -5847,6 +5847,21 @@ bool isolate_huge_page(struct page *page, struct list_head *list)
> >  	return ret;
> >  }
> >  
> > +int get_hwpoison_huge_page(struct page *page, bool *hugetlb)
> > +{
> > +	int ret = 0;
> > +
> > +	*hugetlb = false;
> > +	spin_lock_irq(&hugetlb_lock);
> > +	if (PageHeadHuge(page)) {
> > +		*hugetlb = true;
> > +		if (HPageFreed(page) || HPageMigratable(page))
> > +			ret = get_page_unless_zero(page);
> > +	}
> > +	spin_unlock_irq(&hugetlb_lock);
> > +	return ret;
> > +}
> > +
> >  void putback_active_hugepage(struct page *page)
> >  {
> >  	spin_lock_irq(&hugetlb_lock);
> > diff --git v5.13-rc2/mm/memory-failure.c v5.13-rc2_patched/mm/memory-failure.c
> > index 85ad98c00fd9..353c6177e489 100644
> > --- v5.13-rc2/mm/memory-failure.c
> > +++ v5.13-rc2_patched/mm/memory-failure.c
> > @@ -959,8 +959,14 @@ 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;
> > +	bool hugetlb = false;
> > +
> > +	ret = get_hwpoison_huge_page(head, &hugetlb);
> > +	if (hugetlb)
> > +		return ret;
> >  
> 
> Hello Naoya,
> 
> Thanks for your continued efforts.  However, I believe the race still
> exists.  Unless I am mistaken, it is possible that page is in the hugetlb
> allocation patch and racing with __get_hwpoison_page() as follows:

Hi Mike, thank you for the investigation.

> 
>     CPU0:                           CPU1:
> 
>                                     gather_surplus_pages()
>                                       page = alloc_surplus_huge_page()
> 				      	page = alloc_fresh_huge_page()
> 				      	  page = alloc_buddy_huge_page()
>     memory_failure_hugetlb()
>       get_hwpoison_page(page)
>         __get_hwpoison_page(page)
> 	  get_hwpoison_huge_page()
> 	    /* Note that PageHuge()
> 	       is false, so hugetlb
> 	       not set */
> 	  PageTransHuge(head) false

It seems that PageTransHuge returns true in this race condition because it
simply checks PG_head flag.  But anyway, get_page_unless_zero() is called for
a hugetlb in this race, which is problematic.

> 					  prep_new_huge_page(page)
> 					  /* Now PageHuge() becomes true */
>           get_page_unless_zero(page)
> 
> I am not sure if it is possible to handle this race in the memory error
> code. 

I think that __get_hwpoison_page() might not properly call get_page_unless_zero().
Looking at other callers of this function, most(*) of them are calling it in
the context where a given page is pinned and in-use, and get_page_unless_zero()
is used to detect the race with page freeing.  In the current version,
__get_hwpoison_page() calls get_page_unless_zero() without caring for such an
assumption, which might be the root cause of the race with hugetlb page allocation.

# (*) It seems to me that do_migrate_range() might have the same issue
# around get_page_unless_zero().

So I think of inserting the check to comply with the assumption of
get_hwpoison_huge_page() like below:

        ret = get_hwpoison_huge_page(head, &hugetlb);
        if (hugetlb)
                return ret;

        if (!PageLRU(head) && !__PageMovable(head))
                return 0;

        if (PageTransHuge(head)) {
                ...
        }

        if (get_page_unless_zero(head)) {
                ...
        }

        return 0;

The newly added checks should work to prevent the above race, then get_any_page()
should retry and grab the page properly as a stable hugetlb page.

> I can not think of a way to avoid potentially incrementing the
> ref count on a hugetlb page as it is being created.  There is nothing
> synchronizing this in the hugetlb code.
> 
> When Muchun first proposed a fix to the race, the idea was to catch the
> race in the hugetlb code.  Michal suggested that the memory error code
> be more careful in modifying ref counts.  I would wait a bit to see if
> someone has a good idea how this can be done.  We 'may' need to revisit
> the approach suggested by Muchun.

If the above approach is still broken, let's revisit Muchun's approach.

Thanks,
Naoya Horiguchi