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

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Message-ID: <90d5f255-fb6e-4af8-832f-c0fbf72f7a25@lucifer.local>
Date: Sat, 6 Dec 2025 16:42:35 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: John Hubbard <jhubbard@...dia.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, Dec 05, 2025 at 11:56:32AM -0800, John Hubbard wrote:
> 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.

Ah seems you're right, I just tried a clang/rust build with those dropped and it
worked fine.

Have added to TODO to send a patch to remove those after this one lands, thanks!

Cheers, Lorenzo

>
> > +
> >   #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
>