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

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Message-ID: <b2b2ce39-200c-4dd0-a46a-8d3f184b7d51@nvidia.com>
Date: Fri, 5 Dec 2025 11:56:32 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: 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 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@...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))

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