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

David Hildenbrand (Arm)
2026-03-10T00:00:50Z

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);

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

--
Cheers,

David