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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/17/26 15:11, 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 | 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@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.

Something to ponder about :)

-- 
Cheers,

David