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

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Message-ID: <d8bcc6b5-eb75-4a0f-99a1-295dcb04dfe8@lucifer.local>
Date: Sat, 6 Dec 2025 16:43:57 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Laight <david.laight.linux@...il.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 09:34:49PM +0000, David Laight wrote:
> On Fri, 5 Dec 2025 19:18:56 +0000
> Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:
>
> > On Fri, Dec 05, 2025 at 06:43:42PM +0000, David Laight wrote:
> > > 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.
> >
> > Thanks, checked that and unfortunately that doesn't satisfy sparse :)
> >
> > I don't think it's too crazy to use 1UL << here, just very frustrating (TM)
> > that this is an issue.
>
> I might use some of my copious spare time (ha) to see why BIT() fails.
> I bet it is just too complex for its own good.
> Personally I'm fine with both explicit (1ul << n) and hex constants.
> The latter are definitely most useful if you ever look at hexdumps.

Thanks :) yeah I just didn't want to go down that rabbit hole myself as I seemed
to have the answer and wanted to get it fixed, but obviously am quite curious as
to what on earth is causing that.

>
> At the moment I'm trying to fix bitfield.h so you don't get compile errors
> on lines that are 18KB long.

:)

>
> Found a new version in linux-next - has its own set of new bugs as well
> as more of the old ones.
>
> 	David
>
> >
> > <insert rant about C macros here>
> >
> > Cheers, Lorenzo
>

Cheers, Lorenzo