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

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Message-ID: <aSWBNSbXrWJnpsRl@google.com>
Date: Tue, 25 Nov 2025 10:13:09 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 0/4] initial work on making VMA flags a bitmap

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>