Re: [syzbot] [mm?] WARNING in memory_failure

[jane.chu@oracle.com]

On 9/29/2025 1:15 PM, Zi Yan wrote:
> On 29 Sep 2025, at 14:23, jane.chu@oracle.com wrote:
> 
>> On 9/29/2025 10:49 AM, jane.chu@oracle.com wrote:
>>>
>>> On 9/29/2025 10:29 AM, jane.chu@oracle.com wrote:
>>>>
>>>> On 9/29/2025 4:08 AM, Pankaj Raghav (Samsung) wrote:
>>>>>>
>>>>>> I want to change all the split functions in huge_mm.h and provide
>>>>>> mapping_min_folio_order() to try_folio_split() in truncate_inode_partial_folio().
>>>>>>
>>>>>> Something like below:
>>>>>>
>>>>>> 1. no split function will change the given order;
>>>>>> 2. __folio_split() will no longer give VM_WARN_ONCE when provided new_order
>>>>>> is smaller than mapping_min_folio_order().
>>>>>>
>>>>>> In this way, for an LBS folio that cannot be split to order 0, split
>>>>>> functions will return -EINVAL to tell caller that the folio cannot
>>>>>> be split. The caller is supposed to handle the split failure.
>>>>>
>>>>> IIUC, we will remove warn on once but just return -EINVAL in __folio_split()
>>>>> function if new_order < min_order like this:
>>>>> ...
>>>>>          min_order = mapping_min_folio_order(folio->mapping);
>>>>>          if (new_order < min_order) {
>>>>> -            VM_WARN_ONCE(1, "Cannot split mapped folio below min- order: %u",
>>>>> -                     min_order);
>>>>>              ret = -EINVAL;
>>>>>              goto out;
>>>>>          }
>>>>> ...
>>>>
>>>> Then the user process will get a SIGBUS indicting the entire huge page at higher order -
>>>>                   folio_set_has_hwpoisoned(folio);
>>>>                   if (try_to_split_thp_page(p, false) < 0) {
>>>>                           res = -EHWPOISON;
>>>>                           kill_procs_now(p, pfn, flags, folio);
>>>>                           put_page(p);
>>>>                           action_result(pfn, MF_MSG_UNSPLIT_THP, MF_FAILED);
>>>>                           goto unlock_mutex;
>>>>                   }
>>>>                   VM_BUG_ON_PAGE(!page_count(p), p);
>>>>                   folio = page_folio(p);
>>>>
>>>> the huge page is not usable any way, kind of similar to the hugetlb page situation: since the page cannot be splitted, the entire page is marked unusable.
>>>>
>>>> How about keep the current huge page split code as is, but change the M- F code to recognize that in a successful splitting case, the poisoned page might just be in a lower folio order, and thus, deliver the SIGBUS ?
>>>>
>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>> index a24806bb8e82..342c81edcdd9 100644
>>>> --- a/mm/memory-failure.c
>>>> +++ b/mm/memory-failure.c
>>>> @@ -2291,7 +2291,9 @@ int memory_failure(unsigned long pfn, int flags)
>>>>                    * page is a valid handlable page.
>>>>                    */
>>>>                   folio_set_has_hwpoisoned(folio);
>>>> -               if (try_to_split_thp_page(p, false) < 0) {
>>>> +               ret = try_to_split_thp_page(p, false);
>>>> +               folio = page_folio(p);
>>>> +               if (ret < 0 || folio_test_large(folio)) {
>>>>                           res = -EHWPOISON;
>>>>                           kill_procs_now(p, pfn, flags, folio);
>>>>                           put_page(p);
>>>> @@ -2299,7 +2301,6 @@ int memory_failure(unsigned long pfn, int flags)
>>>>                           goto unlock_mutex;
>>>>                   }
>>>>                   VM_BUG_ON_PAGE(!page_count(p), p);
>>>> -               folio = page_folio(p);
>>>>           }
>>>>
>>>> thanks,
>>>> -jane
>>>
>>> Maybe this is better, in case there are other reason for split_huge_page() to return -EINVAL.
>>>
>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>> index a24806bb8e82..2bfa05acae65 100644
>>> --- a/mm/memory-failure.c
>>> +++ b/mm/memory-failure.c
>>> @@ -1659,9 +1659,10 @@ static int identify_page_state(unsigned long pfn, struct page *p,
>>>    static int try_to_split_thp_page(struct page *page, bool release)
>>>    {
>>>           int ret;
>>> +       int new_order = min_order_for_split(page_folio(page));
>>>
>>>           lock_page(page);
>>> -       ret = split_huge_page(page);
>>> +       ret = split_huge_page_to_list_to_order(page, NULL, new_order);
>>>           unlock_page(page);
>>>
>>>           if (ret && release)
>>> @@ -2277,6 +2278,7 @@ int memory_failure(unsigned long pfn, int flags)
>>>           folio_unlock(folio);
>>>
>>>           if (folio_test_large(folio)) {
>>> +               int ret;
>>>                   /*
>>>                    * The flag must be set after the refcount is bumped
>>>                    * otherwise it may race with THP split.
>>> @@ -2291,7 +2293,9 @@ int memory_failure(unsigned long pfn, int flags)
>>>                    * page is a valid handlable page.
>>>                    */
>>>                   folio_set_has_hwpoisoned(folio);
>>> -               if (try_to_split_thp_page(p, false) < 0) {
>>> +               ret = try_to_split_thp_page(p, false);
>>> +               folio = page_folio(p);
>>> +               if (ret < 0 || folio_test_large(folio)) {
>>>                           res = -EHWPOISON;
>>>                           kill_procs_now(p, pfn, flags, folio);
>>>                           put_page(p);
>>> @@ -2299,7 +2303,6 @@ int memory_failure(unsigned long pfn, int flags)
>>>                           goto unlock_mutex;
>>>                   }
>>>                   VM_BUG_ON_PAGE(!page_count(p), p);
>>> -               folio = page_folio(p);
>>>           }
>>>
>>>           /*
>>> @@ -2618,7 +2621,8 @@ static int soft_offline_in_use_page(struct page *page)
>>>           };
>>>
>>>           if (!huge && folio_test_large(folio)) {
>>> -               if (try_to_split_thp_page(page, true)) {
>>> +               if ((try_to_split_thp_page(page, true)) ||
>>> +                       folio_test_large(page_folio(page))) {
>>>                           pr_info("%#lx: thp split failed\n", pfn);
>>>                           return -EBUSY;
>>>                   }
>>
> 
> What you are proposing here is basically split_huge_page_to_min_order().
> I can add that as a second patch.
> 
>> In soft offline, better to check if (min_order_for_split > 0), no need to split, just return for now ...
> 
> OK. I can do that too.
> 
> Thank you for the input.
> 

That'll be great!  Thank you!

-jane

> 
> Best Regards,
> Yan, Zi