Date:   Thu, 10 Mar 2022 19:46:32 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Yang Shi <shy828301@...il.com>
CC:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] mm/memory-failure.c: fix potential VM_BUG_ON_PAGE in
 split_huge_page_to_list

On 2022/3/9 2:47, Yang Shi wrote:
> On Tue, Mar 8, 2022 at 4:36 AM Miaohe Lin <linmiaohe@...wei.com> wrote:
>>
>> On 2022/3/8 3:53, Yang Shi wrote:
>>> On Sun, Mar 6, 2022 at 11:07 PM Miaohe Lin <linmiaohe@...wei.com> 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 ?
> 

I think about the below code more carefully and I found that this will make
hwpoison_filter can't handle the non-LRU movable pages now. Because non-LRU
movable pages return early now and thus can't reach the hwpoison_filter. This
results in a inconsistent behavior with previous one. So I think the origin
fixup of this patch is more suitable. What do you think?

Thanks.

> I'd prefer this personally. Something like the below (compile test only):
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5444a8ef4867..789e40909ade 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1176,12 +1176,18 @@ void ClearPageHWPoisonTakenOff(struct page *page)
>   * does not return true for hugetlb or device memory pages, so it's assumed
>   * to be called only in the context where we never have such pages.
>   */
> -static inline bool HWPoisonHandlable(struct page *page)
> +static inline bool HWPoisonHandlable(struct page *page, unsigned long flags)
>  {
> - return PageLRU(page) || __PageMovable(page) || is_free_buddy_page(page);
> + bool movable = false;
> +
> + /* Soft offline could mirgate non-LRU movable pages */
> + if ((flags & MF_SOFT_OFFLINE) && __PageMovable(page))
> + movable = true;
> +
> + return movable || PageLRU(page) || is_free_buddy_page(page);
>  }
> 
> -static int __get_hwpoison_page(struct page *page)
> +static int __get_hwpoison_page(struct page *page, unsigned long flags)
>  {
>   struct page *head = compound_head(page);
>   int ret = 0;
> @@ -1196,7 +1202,7 @@ static int __get_hwpoison_page(struct page *page)
>   * for any unsupported type of page in order to reduce the risk of
>   * unexpected races caused by taking a page refcount.
>   */
> - if (!HWPoisonHandlable(head))
> + if (!HWPoisonHandlable(head, flags))
>   return -EBUSY;
> 
>   if (get_page_unless_zero(head)) {
> @@ -1221,7 +1227,7 @@ static int get_any_page(struct page *p, unsigned
> long flags)
> 
>  try_again:
>   if (!count_increased) {
> - ret = __get_hwpoison_page(p);
> + ret = __get_hwpoison_page(p, flags);
>   if (!ret) {
>   if (page_count(p)) {
>   /* We raced with an allocation, retry. */
> @@ -1249,7 +1255,7 @@ static int get_any_page(struct page *p, unsigned
> long flags)
>   }
>   }
> 
> - if (PageHuge(p) || HWPoisonHandlable(p)) {
> + if (PageHuge(p) || HWPoisonHandlable(p, flags)) {
>   ret = 1;
>   } else {
>   /*
> 
>>
>>>
>>> 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@...wei.com>
>>>>>> ---
>>>>>>  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
>>>>
>>>>
>>> .
>>>
>>
> .
> 

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