linux-kernel - Re: [syzbot] [mm?] WARNING in memory

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Message-ID: <CB2E9455-480C-438F-982A-AFCA1051BBDF@nvidia.com>
Date: Mon, 29 Sep 2025 11:20:27 -0400
From: Zi Yan <ziy@...dia.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: David Hildenbrand <david@...hat.com>,
 Luis Chamberlain <mcgrof@...nel.org>,
 syzbot <syzbot+e6367ea2fdab6ed46056@...kaller.appspotmail.com>,
 akpm@...ux-foundation.org, linmiaohe@...wei.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, nao.horiguchi@...il.com,
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] WARNING in memory_failure

On 29 Sep 2025, at 7:08, 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;
> 		}
> ...
>>
>> WDYT?
>>
> I think it should be fine as along as we return an error if someone is
> trying to split < min_order for file-backed folios.
>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index f327d62fc985..e15c3ca07e33 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -387,34 +387,16 @@ int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
>>   * Return: 0: split is successful, otherwise split failed.
>>   */
>>  static inline int try_folio_split(struct folio *folio, struct page *page,
>> -		struct list_head *list)
>> +		struct list_head *list, unsigned int order)
>>  {

OK, I will send a proper patch to fix this. Thanks.

Best Regards,
Yan, Zi