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

[Nikita Kalyazin]

On 23/03/2026 20:22, Ackerley Tng wrote:
> "David Hildenbrand (Arm)" <david at kernel.org> writes:
> 
>> On 3/17/26 15:11, Kalyazin, Nikita wrote:
>>> From: Nikita Kalyazin <kalyazin at amazon.com>
>>>
>>> Move the check for pinning closer to where the result is used.
>>> No functional changes.
>>>
>>> Signed-off-by: Nikita Kalyazin <kalyazin at amazon.com>
>>> ---
>>>   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 at kernel.org>
>>
>> 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.

Ok, I'll leave it as is for now.

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