Re: [PATCH 4/4] mm/memory-failure.c: fix potential VM_BUG_ON_PAGE in split_huge_page_to_list

[Miaohe Lin]

On 2022/3/8 3:53, Yang Shi wrote:
> On Sun, Mar 6, 2022 at 11:07 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> On 2022/3/4 16:28, HORIGUCHI NAOYA(堀口 直也) wrote:
>>> On Mon, Feb 28, 2022 at 10:02:45PM +0800, Miaohe Lin wrote:
>>>> The huge zero page could reach here and if we ever try to split it, the
>>>> VM_BUG_ON_PAGE will be triggered in split_huge_page_to_list(). Also the
>>>> non-lru compound movable pages could be taken for transhuge pages. Skip
>>>> these pages by checking PageLRU because huge zero page isn't lru page as
>>>> non-lru compound movable pages.
>>>
>>> It seems that memory_failure() also fails at get_any_page() with "hwpoison:
>>> unhandlable page" message.
>>>
>>>   [16478.203474] page:00000000b6acdbd1 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1810b4
>>>   [16478.206612] flags: 0x57ffffc0801000(reserved|hwpoison|node=1|zone=2|lastcpupid=0x1fffff)
>>>   [16478.209411] raw: 0057ffffc0801000 fffff11bc6042d08 fffff11bc6042d08 0000000000000000
>>>   [16478.211921] raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
>>>   [16478.214473] page dumped because: hwpoison: unhandlable page
>>>   [16478.216386] Memory failure: 0x1810b4: recovery action for unknown page: Ignored
>>>
>>> We can't handle errors on huge (or normal) zero page, so the current
>>
>> Sorry for confusing commit log again. I should have a coffee before I make this patch.
>> Huge or normal zero page will fail at get_any_page because they're neither HWPoisonHandlable
>> nor PageHuge.
>>
>>> behavior seems to me more suitable than "unsplit thp".
>>>
>>> Or if you have some producer to reach the following path with huge zero
>>> page, could you share it?
>>>
>>
>> What I mean is that non-lru movable compound page can reach here unexpected because __PageMovable(page)
>> is handleable now. So get_any_page could succeed to grab the page refcnt. And since it's compound page,
>> it will go through the split_huge_page_to_list because PageTransHuge checks PageHead(page) which can also
>> be true for compound page. But this type of pages is unexpected for split_huge_page_to_list.
> 
> Can we really handle non-LRU movable pages in memory failure
> (uncorrectable errors)? Typically they are balloon, zsmalloc, etc.
> Assuming we run into a base (4K) non-LRU movable page, we could reach
> as far as identify_page_state(), it should not fall into any category
> except me_unknown. So it seems we could just simply make it
> unhandlable.

There is the comment from memory_failure:
	/*
	 * We ignore non-LRU pages for good reasons.
	 * - PG_locked is only well defined for LRU pages and a few others
	 * - to avoid races with __SetPageLocked()
	 * - to avoid races with __SetPageSlab*() (and more non-atomic ops)
	 * The check (unnecessarily) ignores LRU pages being isolated and
	 * walked by the page reclaim code, however that's not a big loss.
	 */

So we could not handle non-LRU movable pages.

What do you mean is something like below?

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 5444a8ef4867..d80dbe0f20b6 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1784,6 +1784,13 @@ int memory_failure(unsigned long pfn, int flags)
                }
        }

+       if (__PageMovable(hpage)) {
+               put_page(p);
+               action_result(pfn, MF_MSG_MOVALBE_PAGE, MF_IGNORED);
+               res = -EBUSY;
+               goto unlock_mutex;
+       }
+
        if (PageTransHuge(hpage)) {
                /*
                 * The flag must be set after the refcount is bumped


i.e. Simply make non-LRU movable pages unhandlable ?

> 
> But it should be handlable for soft-offline since it could be migrated.
> 

Yes, non-LRU movable pages can be simply migrated.

Many thanks.

> 
>> Does this make sense for you? Thanks Naoya.
>>
>>> Thanks,
>>> Naoya Horiguchi
>>>
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>> ---
>>>>  mm/memory-failure.c | 14 ++++++++++++++
>>>>  1 file changed, 14 insertions(+)
>>>>
>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>> index 23bfd809dc8c..ac6492e36978 100644
>>>> --- a/mm/memory-failure.c
>>>> +++ b/mm/memory-failure.c
>>>> @@ -1792,6 +1792,20 @@ int memory_failure(unsigned long pfn, int flags)
>>>>      }
>>>>
>>>>      if (PageTransHuge(hpage)) {
>>>> +            /*
>>>> +             * The non-lru compound movable pages could be taken for
>>>> +             * transhuge pages. Also huge zero page could reach here
>>>> +             * and if we ever try to split it, the VM_BUG_ON_PAGE will
>>>> +             * be triggered in split_huge_page_to_list(). Skip these
>>>> +             * pages by checking PageLRU because huge zero page isn't
>>>> +             * lru page as non-lru compound movable pages.
>>>> +             */
>>>> +            if (!PageLRU(hpage)) {
>>>> +                    put_page(p);
>>>> +                    action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
>>>> +                    res = -EBUSY;
>>>> +                    goto unlock_mutex;
>>>> +            }
>>>>              /*
>>>>               * The flag must be set after the refcount is bumped
>>>>               * otherwise it may race with THP split.
>>>> --
>>>> 2.23.0
>>
>>
> .
>