Re: [PATCH v11 05/16] mm/gup: drop local variable in gup_fast_folio_allowed

[Ackerley Tng]

"David Hildenbrand (Arm)" <david@xxxxxxxxxx> writes:

> On 3/17/26 15:11, Kalyazin, Nikita wrote:
>> From: Nikita Kalyazin <kalyazin@xxxxxxxxxx>
>>
>> Move the check for pinning closer to where the result is used.
>> No functional changes.
>>
>> Signed-off-by: Nikita Kalyazin <kalyazin@xxxxxxxxxx>
>> ---
>>  mm/gup.c | 23 ++++++++++++-----------
>>  1 file changed, 12 insertions(+), 11 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 5856d35be385..869d79c8daa4 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -2737,18 +2737,9 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
>>   */
>>  static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
>>  {
>> -	bool reject_file_backed = false;
>>  	struct address_space *mapping;
>>  	unsigned long mapping_flags;
>>
>> -	/*
>> -	 * If we aren't pinning then no problematic write can occur. A long term
>> -	 * pin is the most egregious case so this is the one we disallow.
>> -	 */
>> -	if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) ==
>> -	    (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
>> -		reject_file_backed = true;
>> -
>>  	/* We hold a folio reference, so we can safely access folio fields. */
>>  	if (WARN_ON_ONCE(folio_test_slab(folio)))
>>  		return false;
>> @@ -2793,8 +2784,18 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
>>  	 */
>>  	if (secretmem_mapping(mapping))
>>  		return false;
>> -	/* The only remaining allowed file system is shmem. */
>> -	return !reject_file_backed || shmem_mapping(mapping);
>> +
>> +	/*
>> +	 * If we aren't pinning then no problematic write can occur. A writable
>> +	 * long term pin is the most egregious case, so this is the one we
>> +	 * allow only for ...
>> +	 */
>> +	if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
>> +	    (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
>> +		return true;
>> +
>> +	/* ... hugetlb (which we allowed above already) and shared memory. */
>> +	return shmem_mapping(mapping);
>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>
> I'm wondering if it would be a good idea to check for a hugetlb mapping
> here instead of having the folio_test_hugetlb() check above.
>

I think it's nice that hugetlb folios are determined immediately to be
eligible for GUP-fast regardless of whether the folio is file-backed or
not.

> Something to ponder about :)
>
> --
> Cheers,
>
> David