Re: [PATCH] mm: avoid use of BIT() macro for initialising VMA flags

[John Hubbard]

On 12/5/25 9:50 AM, Lorenzo Stoakes wrote:
> Commit 2b6a3f061f11 ("mm: declare VMA flags by bit") significantly changed
> how VMA flags are declared, utilising an enum of VMA bit values and
> ifdef-fery VM_xxx flag declarations via macro.
>
> As part of this change, it uses INIT_VM_FLAG() to define VM_xxx flags from
> the newly introduced VMA bit numbers.
>
> However, use of this macro results in apparently unfortunate macro
> expansion and resulted in a performance degradation.This appears to be due
> to the (__force int), which is required for the sparse typechecking to
> work.
>
> Avoid macro expansion issues by simply using 1UL << bitnum.
>
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202512041634.150c7e4f-lkp@xxxxxxxxx
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
>
> Andrew - note I've referenced the linux-next commit number above, could you
> replace with the upstream commit hash once your PR is taken? Thanks!
>
>   include/linux/mm.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a2f38fb68840..c4438b30c140 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -395,7 +395,8 @@ enum {
>   #undef DECLARE_VMA_BIT
>   #undef DECLARE_VMA_BIT_ALIAS
>
> -#define INIT_VM_FLAG(name) BIT((__force int) VMA_ ## name ## _BIT)
> +#define INIT_VM_FLAG(name) (1UL << (__force int)(VMA_ ## name ## _BIT))

OK, so now maybe we don't need all of the rust/bindings/bindings_helper.h
changes? Those were because Rust's bindgen doesn't properly handle
nested macros, as I recall.

> +
>   #define VM_READ		INIT_VM_FLAG(READ)
>   #define VM_WRITE	INIT_VM_FLAG(WRITE)
>   #define VM_EXEC		INIT_VM_FLAG(EXEC)
> --
> 2.52.0

thanks,
--
John Hubbard