Re: [Patch v3 2/2] mm/huge_memory: merge uniform_split_supported() and non_uniform_split_supported()

[Zi Yan <ziy@nvidia.com>]

On 16 Nov 2025, at 20:22, Wei Yang wrote:

> On Thu, Nov 06, 2025 at 03:41:55AM +0000, Wei Yang wrote:
>> The functions uniform_split_supported() and
>> non_uniform_split_supported() share significantly similar logic.
>>
>> The only functional difference is that uniform_split_supported()
>> includes an additional check on the requested @new_order.
>>
>> The reason for this check comes from the following two aspects:
>>
>>  * some file system or swap cache just supports order-0 folio
>>  * the behavioral difference between uniform/non-uniform split
>>
>> The behavioral difference between uniform split and non-uniform:
>>
>>  * uniform split splits folio directly to @new_order
>>  * non-uniform split creates after-split folios with orders from
>>    folio_order(folio) - 1 to new_order.
>>
>> This means for non-uniform split or !new_order split we should check the
>> file system and swap cache respectively.
>>
>> This commit unifies the logic and merge the two functions into a single
>> combined helper, removing redundant code and simplifying the split
>> support checking mechanism.
>>
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
>>
> [...]
>> -/* See comments in non_uniform_split_supported() */
>> -bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>> -		bool warns)
>> -{
>> -	if (folio_test_anon(folio)) {
>> -		VM_WARN_ONCE(warns && new_order == 1,
>> -				"Cannot split to order-1 folio");
>> -		if (new_order == 1)
>> -			return false;
>> -	} else  if (new_order) {
>> +	} else if (split_type == SPLIT_TYPE_NON_UNIFORM || new_order) {
>> 		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>> 		    !mapping_large_folio_support(folio->mapping)) {
>
> After re-scan the code, I found we may have a NULL pointer dereference here.
>
> We bail out if folio->mapping == NULL in __folio_split(), which means it is
> possible to be NULL. But we access mapping->flags here.
>
> Looks there is no bug report yet, so I am not sure it worth a separate fix to
> original code.

Probably because the race is small, but a fix is still needed.
Likely commit 6a50c9b512f7 ("mm: huge_memory: fix misused
mapping_large_folio_support() for anon folios") introduced it, but please
double check.

>
>> +			/*
>> +			 * We can always split a folio down to a single page
>> +			 * (new_order == 0) uniformly.
>> +			 *
>> +			 * For any other scenario
>> +			 *   a) uniform split targeting a large folio
>> +			 *      (new_order > 0)
>> +			 *   b) any non-uniform split
>> +			 * we must confirm that the file system supports large
>> +			 * folios.
>> +			 *
>> +			 * Note that we might still have THPs in such
>> +			 * mappings, which is created from khugepaged when
>> +			 * CONFIG_READ_ONLY_THP_FOR_FS is enabled. But in that
>> +			 * case, the mapping does not actually support large
>> +			 * folios properly.
>> +			 */
>> 			VM_WARN_ONCE(warns,
>> 				"Cannot split file folio to non-0 order");
>> 			return false;
>> 		}
>> 	}
>>
>> -	if (new_order && folio_test_swapcache(folio)) {
>> +	/*
>> +	 * swapcache folio could only be split to order 0
>> +	 *
>> +	 * non-uniform split creates after-split folios with orders from
>> +	 * folio_order(folio) - 1 to new_order, making it not suitable for any
>> +	 * swapcache folio split. Only uniform split to order-0 can be used
>> +	 * here.
>> +	 */
>> +	if ((split_type == SPLIT_TYPE_NON_UNIFORM || new_order) && folio_test_swapcache(folio)) {
>> 		VM_WARN_ONCE(warns,
>> 			"Cannot split swapcache folio to non-0 order");
>> 		return false;
>> @@ -3794,11 +3787,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>> 	if (new_order >= old_order)
>> 		return -EINVAL;
>>
>> -	if (split_type == SPLIT_TYPE_UNIFORM && !uniform_split_supported(folio, new_order, true))
>> -		return -EINVAL;
>> -
>> -	if (split_type == SPLIT_TYPE_NON_UNIFORM &&
>> -	    !non_uniform_split_supported(folio, new_order, true))
>> +	if (!folio_split_supported(folio, new_order, split_type, /* warn = */ true))
>> 		return -EINVAL;
>>
>> 	is_hzp = is_huge_zero_folio(folio);
>> -- 
>> 2.34.1
>
> -- 
> Wei Yang
> Help you, Help me


Best Regards,
Yan, Zi