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

[David Laight <david.laight.linux@gmail.com>]

On Fri,  5 Dec 2025 17:50:37 +0000
Lorenzo Stoakes <lorenzo.stoakes@oracle.com> 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.

Does sparse complain if you just add 0? As in:
#define INIT_VM_FLAG(name) BIT(VMA_ ## name ## _BIT + 0u)

That should change the type without affecting what BIT() expands to.

	David

> 
> Avoid macro expansion issues by simply using 1UL << bitnum.
> 
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202512041634.150c7e4f-lkp@intel.com
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
> 
> 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))
> +
>  #define VM_READ		INIT_VM_FLAG(READ)
>  #define VM_WRITE	INIT_VM_FLAG(WRITE)
>  #define VM_EXEC		INIT_VM_FLAG(EXEC)
> --
> 2.52.0
>