linux-kernel - Re: [PATCH v3 0/4] initial work on making VMA flags a bitmap

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Message-ID: <e7af6426-56cc-4615-9696-019987748f51@lucifer.local>
Date: Tue, 25 Nov 2025 10:25:27 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <muchun.song@...ux.dev>,
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Subject: Re: [PATCH v3 0/4] initial work on making VMA flags a bitmap

On Tue, Nov 25, 2025 at 10:13:09AM +0000, Alice Ryhl wrote:
> On Tue, Nov 25, 2025 at 10:00:58AM +0000, Lorenzo Stoakes wrote:
> > We are in the rather silly situation that we are running out of VMA flags
> > as they are currently limited to a system word in size.
> >
> > This leads to absurd situations where we limit features to 64-bit
> > architectures only because we simply do not have the ability to add a flag
> > for 32-bit ones.
> >
> > This is very constraining and leads to hacks or, in the worst case, simply
> > an inability to implement features we want for entirely arbitrary reasons.
> >
> > This also of course gives us something of a Y2K type situation in mm where
> > we might eventually exhaust all of the VMA flags even on 64-bit systems.
> >
> > This series lays the groundwork for getting away from this limitation by
> > establishing VMA flags as a bitmap whose size we can increase in future
> > beyond 64 bits if required.
> >
> > This is necessarily a highly iterative process given the extensive use of
> > VMA flags throughout the kernel, so we start by performing basic steps.
> >
> > Firstly, we declare VMA flags by bit number rather than by value, retaining
> > the VM_xxx fields but in terms of these newly introduced VMA_xxx_BIT
> > fields.
> >
> > While we are here, we use sparse annotations to ensure that, when dealing
> > with VMA bit number parameters, we cannot be passed values which are not
> > declared as such - providing some useful type safety.
> >
> > We then introduce an opaque VMA flag type, much like the opaque mm_struct
> > flag type introduced in commit bb6525f2f8c4 ("mm: add bitmap mm->flags
> > field"), which we establish in union with vma->vm_flags (but still set at
> > system word size meaning there is no functional or data type size change).
> >
> > We update the vm_flags_xxx() helpers to use this new bitmap, introducing
> > sensible helpers to do so.
> >
> > This series lays the foundation for further work to expand the use of
> > bitmap VMA flags and eventually eliminate these arbitrary restrictions.
>
> LGTM from Rust perspective.
>
> Acked-by: Alice Ryhl <aliceryhl@...gle.com>

Thanks! :)