Re: [PATCH v10 04/15] mm/gup: drop local variable in gup_fast_folio_allowed

[Nikita Kalyazin <kalyazin@amazon.com>]

On 05/03/2026 19:07, David Hildenbrand (Arm) wrote:
> On 1/26/26 17:47, Kalyazin, Nikita wrote:
>> From: Nikita Kalyazin <kalyazin@amazon.com>
>>
>> Move the check for pinning closer to where the result is used.
>> No functional changes.
>>
>> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
>> ---
>>   mm/gup.c | 19 ++++++++-----------
>>   1 file changed, 8 insertions(+), 11 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 9cad53acbc99..e72dacce3e34 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 +2784,14 @@ 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 long term
>> +      * pin is the most egregious case so this is the one we disallow.
>> +      * Also check the only remaining allowed file system - shmem.
>> +      */
>> +     return (flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
>> +         (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE) || shmem_mapping(mapping);
> 
> Best to keep this split and a bit more readable.
> 
> /*
>   * 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);

Makes sense, thank you!

> 
>>   }
>>
>>   static void __maybe_unused gup_fast_undo_dev_pagemap(int *nr, int nr_start,
> 
> 
> --
> Cheers,
> 
> David