linux-kernel - Re: [PATCH 4/4] mm: introduce and use VMA flag test helpers

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Message-ID: <20251029192214.GT760669@ziepe.ca>
Date: Wed, 29 Oct 2025 16:22:14 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>,
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Subject: Re: [PATCH 4/4] mm: introduce and use VMA flag test helpers

On Wed, Oct 29, 2025 at 05:49:38PM +0000, Lorenzo Stoakes wrote:
> We introduce vma_flags_test() and vma_test() (the latter operating on a
> VMA, the former on a pointer to a vma_flags_t value).
> 
> It's useful to have both, as many functions modify a local VMA flags
> variable before setting the VMA flags to this value.

Hmm, sure would be nice to not have this inconsistency though.

It is a bit wordy but with the C preprocessor we can make this work:

struct vm_flags_t {DECLARE_BITMAP(..)};

void func(..)
{
   struct vm_flags_t flags = OR_VMA_FLAGS(VMA_READ_BIT, VMA_WRITE_BIT);

   flags = vm_flags_or(flags, OR_VMA_FLAGS(VMA_MAYREAD_BIT, VMA_MAYWRITE_BIT);
}

Where OR_VMA_FLAGS's OR's together its __VA_ARGS__ and returns a struct vm_flags_t.

Would that be interesting? Eliminate the inconsistency?

eg

https://stackoverflow.com/questions/77244843/c-macro-to-bitwise-or-together-a-variable-number-of-arguments-lightweight-solut

Or other similar solutions.

The compiler is pretty smart so this would all fold away to very
few instructions.

Then everything only works with _BIT and we don't have the special
first word situation.

Jason