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

[jane.chu@oracle.com]

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