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

jane.chu
2025-09-30T04:36:31Z

On 9/29/2025 7:51 PM, Miaohe Lin wrote:
> On 2025/9/30 2:23, 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;
>>>                  }
>>
>> In soft offline, better to check if (min_order_for_split > 0), no need to split, just return for now ...
>
> I might be miss something but why we have to split it? Could we migrate the whole thp or folio with min_order instead?

The soft offline code was originally written with the assumption that
only 1 base page will be offlined. With the recent introduction of
min_order, it might quietly offline multiple pages, is that a desirable
thing? Recently we see patch aimed at not soft offlining hugetlb page
while okay to offline 4K page though.

What do you think?

thanks,
-jane

>
> Thanks.
> .