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

[jane.chu@oracle.com]

On 9/29/2025 11:31 PM, Miaohe Lin wrote:
> On 2025/9/30 12:35, jane.chu@oracle.com wrote:
>>
>>
>> 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.
> 
> Yes, only page corresponding to parameter @pfn of soft_offline_page() will be offlined.
> 
>>
>> With the recent introduction of min_order, it might quietly offline multiple pages, is that a desirable thing?
> 
> I don't think so. Even if try_to_split_thp_page splits folio into smaller one with min_order, page_handle_poison()
> will put back the folio into buddy after migrate_pages, set the hwpoisoned flag to raw error page and hold the extra
> refcnt. So only raw error page will be offlined while other sub-pages will be put back into buddy.
> Or am I miss something?

The thing is that the non-poisoned subpages are also migrated away, 
they're no longer available for the user process.

thanks,
-jane

> 
> Thanks.
> .