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 <20230125162810.ec222773d13cd26c55991fde@linux-foundation.org>
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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 25 Jan 2023 16:56:17 -0800
Message-ID: <CAJuCfpFWTNpz7LB+931Gc+yYwBq3-y+_doH2WdtjhTGnxLxvig@mail.gmail.com>
Subject: Re: [PATCH v3 2/7] mm: introduce vma->vm_flags wrapper functions
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     michel@lespinasse.org, jglisse@google.com, mhocko@suse.com,
        vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net,
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On Wed, Jan 25, 2023 at 4:28 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 25 Jan 2023 15:35:49 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
>
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -491,7 +491,15 @@ struct vm_area_struct {
> >        * See vmf_insert_mixed_prot() for discussion.
> >        */
> >       pgprot_t vm_page_prot;
> > -     unsigned long vm_flags;         /* Flags, see mm.h. */
> > +
> > +     /*
> > +      * Flags, see mm.h.
> > +      * To modify use {init|reset|set|clear|mod}_vm_flags() functions.
> > +      */
> > +     union {
> > +             const vm_flags_t vm_flags;
> > +             vm_flags_t __private __vm_flags;
> > +     };
>
> Typically when making a change like this we'll rename the affected
> field/variable/function/etc.  This will reliably and deliberately break
> unconverted usage sites.
>
> This const trick will get us partway there, by breaking setters.  But
> renaming it will break both setters and getters.

My intent here is to break setters but to allow getters to keep
reading vma->vm_flags directly. We could provide get_vm_flags() and
convert all getters as well but it would introduce a huge additional
churn (800+ hits) with no obvious benefits, I think. Does that clarify
the intent of this trick?

>
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