Message-ID: <7ac64213-26f9-470c-bf6a-abd1c0f6c83d@lucifer.local>
Date: Tue, 20 Jan 2026 16:50:21 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Jason Gunthorpe <jgg@...dia.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 05:44:29PM +0100, Arnd Bergmann wrote:
> On Tue, Jan 20, 2026, at 17:22, Lorenzo Stoakes wrote:
> > On Tue, Jan 20, 2026 at 05:00:28PM +0100, Arnd Bergmann wrote:
> >> On Tue, Jan 20, 2026, at 16:10, Lorenzo Stoakes wrote:
> >> >
> >> > It strikes me that the key optimisation here is the inlining, now if the issue
> >> > is that ye olde compiler might choose not to inline very small functions (seems
> >> > unlikely) we could always throw in an __always_inline?
> >>
> >> I can think of three specific things going wrong with structures passed
> >> by value:
> >
> > I mean now you seem to be talking about it _in general_ which, _in theory_,
> > kills the whole concept of bitmap VMA flags _altogether_ really, or at
> > least any workable version of them.
>
> No, what I'm saying is "understand what the pitfalls are", not
> "don't do it". I think that is what Jason was also getting at.
>
>      Arnd

Ack sure and your input is appreciated :) It's important to kick the tyres
and be aware of possible issues.

Actually I think now I understand where Jason's coming from - the by-value
cases will be const value for the most part - which should make life MUCH
easier for the compiler and avoid a lot of the issues you raised.

So _hopefully_ we're mitigated. Again as I said, in cases where we might
not be, I will take action to figure out workarounds.

I'm excited by the proposed approach in general (+ again thanks to Jason to
opening my eyes to the possibility in the first place), so perhaps a
_little_ defensive, as it allows for a like-for-like replacement generally
which should HUGELY speed up + simplify the transition :)

Cheers, Lorenzo

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