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

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Message-ID: <20251205214940.211a2de2@pumpkin>
Date: Fri, 5 Dec 2025 21:49:40 +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 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 :)

Oh - it is that __bitwise that causes grief.

> I don't think it's too crazy to use 1UL << here, just very frustrating (TM)
> that this is an issue.

Did you try getting DECLARE_VMA_BIT to define both the bit number and the
bit flag and put them both into the anonymous enum?
Or are there other reasons for not doing that?

> 
> <insert rant about C macros here>

Add rant about the compiler thinking anon enums are doing anything other
than defining constants.

	David

> 
> Cheers, Lorenzo