It clarifies that folio_split_supported() does not check folio->mapping and
can dereference it.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index efea42d68157..15e555f1b85d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 	return 0;
 }
 
+/**
+ * folio_split_supported() - check if a folio can be split to a given order
+ * @folio: folio to be split
+ * @new_order: the smallest order of the after split folios (since buddy
+ *             allocator like split generates folios with orders from @folio's
+ *             order - 1 to new_order).
+ * @split_type: uniform or non-uniform split
+ * @warns: whether gives warnings or not for the checks in the function
+ *
+ * folio_split_supported() checks if @folio can be split to @new_order using
+ * @split_type method.
+ *
+ * Context: Caller must make sure folio->mapping is not NULL, since the
+ *          function does not check it and can dereference folio->mapping
+ * Return: true - @folio can be split to @new_order, false - @folio cannot be
+ * split
+ */
 bool folio_split_supported(struct folio *folio, unsigned int new_order,
 		enum split_type split_type, bool warns)
 {
-- 
2.51.0

On 11/20/25 04:59, Zi Yan wrote:
> It clarifies that folio_split_supported() does not check folio->mapping and
> can dereference it.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>   mm/huge_memory.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index efea42d68157..15e555f1b85d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>   	return 0;
>   }
>   
> +/**
> + * folio_split_supported() - check if a folio can be split to a given order
> + * @folio: folio to be split
> + * @new_order: the smallest order of the after split folios (since buddy
> + *             allocator like split generates folios with orders from @folio's
> + *             order - 1 to new_order).
> + * @split_type: uniform or non-uniform split
> + * @warns: whether gives warnings or not for the checks in the function
> + *
> + * folio_split_supported() checks if @folio can be split to @new_order using
> + * @split_type method.
> + *
> + * Context: Caller must make sure folio->mapping is not NULL, since the
> + *          function does not check it and can dereference folio->mapping

Only for anon folios. Also, I would drop the detail about dereference.

I guess we really need the folio lock to prevent concurrent truncation.

Maybe something like:

"The folio must be locked. For non-anon folios, the caller must make 
sure that folio->mapping is not NULL (e.g., not truncated)."

-- 
Cheers

David

On 20 Nov 2025, at 4:27, David Hildenbrand (Red Hat) wrote:

> On 11/20/25 04:59, Zi Yan wrote:
>> It clarifies that folio_split_supported() does not check folio->mapping and
>> can dereference it.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   mm/huge_memory.c | 17 +++++++++++++++++
>>   1 file changed, 17 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index efea42d68157..15e555f1b85d 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>   	return 0;
>>   }
>>  +/**
>> + * folio_split_supported() - check if a folio can be split to a given order
>> + * @folio: folio to be split
>> + * @new_order: the smallest order of the after split folios (since buddy
>> + *             allocator like split generates folios with orders from @folio's
>> + *             order - 1 to new_order).
>> + * @split_type: uniform or non-uniform split
>> + * @warns: whether gives warnings or not for the checks in the function
>> + *
>> + * folio_split_supported() checks if @folio can be split to @new_order using
>> + * @split_type method.
>> + *
>> + * Context: Caller must make sure folio->mapping is not NULL, since the
>> + *          function does not check it and can dereference folio->mapping
>
> Only for anon folios. Also, I would drop the detail about dereference.

OK.

>
> I guess we really need the folio lock to prevent concurrent truncation.
>
> Maybe something like:
>
> "The folio must be locked. For non-anon folios, the caller must make sure that folio->mapping is not NULL (e.g., not truncated)."

Sure. Do you think it is worth adding VM_WARN_ONCE_ON(!folio_test_locked);
and VM_WARN_ONCE_ON(!folio->mapping); ?

Best Regards,
Yan, Zi

On 11/20/25 15:48, Zi Yan wrote:
> On 20 Nov 2025, at 4:27, David Hildenbrand (Red Hat) wrote:
> 
>> On 11/20/25 04:59, Zi Yan wrote:
>>> It clarifies that folio_split_supported() does not check folio->mapping and
>>> can dereference it.
>>>
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> ---
>>>    mm/huge_memory.c | 17 +++++++++++++++++
>>>    1 file changed, 17 insertions(+)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index efea42d68157..15e555f1b85d 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>>    	return 0;
>>>    }
>>>   +/**
>>> + * folio_split_supported() - check if a folio can be split to a given order
>>> + * @folio: folio to be split
>>> + * @new_order: the smallest order of the after split folios (since buddy
>>> + *             allocator like split generates folios with orders from @folio's
>>> + *             order - 1 to new_order).
>>> + * @split_type: uniform or non-uniform split
>>> + * @warns: whether gives warnings or not for the checks in the function
>>> + *
>>> + * folio_split_supported() checks if @folio can be split to @new_order using
>>> + * @split_type method.
>>> + *
>>> + * Context: Caller must make sure folio->mapping is not NULL, since the
>>> + *          function does not check it and can dereference folio->mapping
>>
>> Only for anon folios. Also, I would drop the detail about dereference.
> 
> OK.
> 
>>
>> I guess we really need the folio lock to prevent concurrent truncation.
>>
>> Maybe something like:
>>
>> "The folio must be locked. For non-anon folios, the caller must make sure that folio->mapping is not NULL (e.g., not truncated)."
> 
> Sure. Do you think it is worth adding VM_WARN_ONCE_ON(!folio_test_locked);
> and VM_WARN_ONCE_ON(!folio->mapping); ?

Makes sense. Or we allow !folio->mapping, return false and do something 
like the following. Still wondering how we could handle that case better.

if (!folio_split_supported(folio)) {
	if (folio_split_temporarily_unsupported(folio))
		return -EBUSY;
	return -EINVAL;
}

hmmmm

-- 
Cheers

David

On 11/20/25 14:59, Zi Yan wrote:
> It clarifies that folio_split_supported() does not check folio->mapping and
> can dereference it.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  mm/huge_memory.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index efea42d68157..15e555f1b85d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  	return 0;
>  }
>  
> +/**
> + * folio_split_supported() - check if a folio can be split to a given order
> + * @folio: folio to be split
> + * @new_order: the smallest order of the after split folios (since buddy
> + *             allocator like split generates folios with orders from @folio's
> + *             order - 1 to new_order).
> + * @split_type: uniform or non-uniform split
> + * @warns: whether gives warnings or not for the checks in the function
> + *
> + * folio_split_supported() checks if @folio can be split to @new_order using
> + * @split_type method.
> + *
> + * Context: Caller must make sure folio->mapping is not NULL, since the
> + *          function does not check it and can dereference folio->mapping
> + * Return: true - @folio can be split to @new_order, false - @folio cannot be
> + * split
> + */
>  bool folio_split_supported(struct folio *folio, unsigned int new_order,
>  		enum split_type split_type, bool warns)
>  {

Looks good!

Reviewed-by: Balbir Singh <balbirs@nvidia.com>