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

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Message-ID: <20251205184342.2cfcc73e@pumpkin>
Date: Fri, 5 Dec 2025 18:43:42 +0000
From: David Laight <david.laight.linux@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand
 <david@...nel.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, Vlastimil
 Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan
 <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, oliver.sang@...el.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: avoid use of BIT() macro for initialising VMA flags

On Fri,  5 Dec 2025 17:50:37 +0000
Lorenzo Stoakes <lorenzo.stoakes@...cle.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@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202512041634.150c7e4f-lkp@intel.com
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.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
>