linux-kernel - Re: [PATCH RESEND 09/12] mm: make vm_area_desc utilise vma_flags

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Message-ID: <20260120133619.GZ1134360@nvidia.com>
Date: Tue, 20 Jan 2026 09:36:19 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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Subject: Re: [PATCH RESEND 09/12] mm: make vm_area_desc utilise vma_flags_t
 only

On Tue, Jan 20, 2026 at 09:46:05AM +0000, Lorenzo Stoakes wrote:
> On Mon, Jan 19, 2026 at 07:14:03PM -0400, Jason Gunthorpe wrote:
> > On Mon, Jan 19, 2026 at 09:19:11PM +0000, Lorenzo Stoakes wrote:
> > > +static inline bool is_shared_maywrite(vma_flags_t flags)
> > > +{
> >
> > I'm not sure it is ideal to pass this array by value? Seems like it
> > might invite some negative optimizations since now the compiler has to
> > optimze away a copy too.
> 
> I really don't think so? This is inlined and thus collapses to a totally
> standard vma_flags_test_all() which passes by value anyway.

> Do you have specific examples or evidence the compiler will optimise poorly here
> on that basis as compared to pass by reference? And pass by reference would
> necessitate:

I've recently seen enough cases of older compilers and other arches
making weird choices to be a little concerened. In the above case
there is no reason not to use a const pointer (and indeed that would
be the expected idomatic kernel style), so why take chances is my
thinking.

Jason