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

[jane.chu@oracle.com]

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 ...

thanks,
-jane

> 
> 
> thanks,
> -jane
> 
>