Message-ID: <wqtf6zwf72i5mmq4jxzea5lg7nziinqrm7eyou6n76ticah4py@ju4i4mndzsr2>
Date: Wed, 28 Jan 2026 15:50:19 +0000
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Yury Norov <ynorov@...dia.com>, 
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Subject: Re: [PATCH v2 00/13] mm: add bitmap VMA flag helpers and convert all
 mmap_prepare to use them

On Wed, Jan 28, 2026 at 09:33:44AM +0000, Lorenzo Stoakes wrote:
> On Tue, Jan 27, 2026 at 04:36:44PM -0500, Yury Norov wrote:
> > On Tue, Jan 27, 2026 at 02:40:03PM +0000, Lorenzo Stoakes wrote:
> > > On Tue, Jan 27, 2026 at 08:53:44AM -0500, Yury Norov wrote:
> > > > On Thu, Jan 22, 2026 at 04:06:09PM +0000, Lorenzo Stoakes wrote:
> >
> > ...
> >
> > > > Even if you expect adding more flags, u128 would double your capacity,
> > > > and people will still be able to use language-supported operation on
> > > > the bits in flag. Which looks simpler to me...
> > >
> > > u128 isn't supported on all architectures, VMA flags have to have absolutely
> >
> > What about big integers?
> >
> >         typedef unsigned _BitInt(VMA_FLAGS_COUNT) vma_flags_t
> 
> There is no use of _BitInt anywhere in the kernel. That seems to be a
> C23-only feature with limited compiler support that we simply couldn't use
> yet.
> 
> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3639r0.html tells
> me that it's supported in clang 16+ and gcc 14+.
> 
> We cannot put such a restriction on compilers in the kernel, obviously.
> 
> >
> > > We want to be able to arbitrarily extend this as we please in the future. So
> > > using u64 wouldn't buy us _anything_ except getting the 32-bit kernels in line.
> >
> > So enabling 32-bit arches is a big deal, even if it's a temporary
> > solution. Again, how many flags in your opinion are blocked because of
> > 32-bit integer limitation? How soon 64-bit capacity will get fully
> > used?
> 
> In my opinion? I'm not sure where my opinion comes into this? There are 43 VMA
> flags and 32-bits available in 32-bit kernels.
> 
> As I said to you before Yury, when adding new flags you have to add a whole
> load of mess of #ifdef CONFIG_64BIT ... #endif etc. around things that have
> nothing to do with 64-bit vs 32-bit architecture as a result.
> 
> It's a mess, we've run out.
> 
> Also something that might not have occurred to you - there is a chilling
> effect of limited VMA flag availability - the bar to adding flags is
> higher, and features that might have used VMA flags but need general kernel
> support (incl. 32-bit) have to find other ways to store state like this.
> 

For the record, I fully agree with all of the points you made. I don't think
it makes sense to hold this change back waiting for a feature that right
now is relatively unobtainable (also IIRC the ABI around _BitInt was a bit
unstable and confusing in general, I don't know if that changed).

The goals are to:
1) get more than sizeof(unsigned long) * 8 flags so we don't have to uglify
and gatekeep things behind 64-bit. Also letting us use VMA flags more freely.
2) not cause any performance/codegen regression

Yes, the current patchset (and the current state of things too) uglifies
things a bit, but it also provides things like type safety which are sorely
needed here. And which 128-bit integers, or N-bit integers would not provide.

And if any of the above suddenly become available to us in the future, it
will be trivial to change because the VMA flags types will be fully
encapsulated with proper accessors.

Or perhaps we'll rewrite it all in rust by the end of the decade and this is
all a moot point, who knows ;)

-- 
Pedro

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