linux-kernel - Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always lazily freeable mappings

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Date:   Fri, 6 Jan 2023 14:06:51 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Yann Droneaud <ydroneaud@...eya.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, tglx@...utronix.de,
        linux-crypto@...r.kernel.org, linux-api@...r.kernel.org,
        x86@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        "Carlos O'Donell" <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Christian Brauner <brauner@...nel.org>, linux-mm@...ck.org
Subject: Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

On Fri, Jan 6, 2023 at 1:42 PM Matthew Wilcox <willy@...radead.org> wrote:
>
>
> I'd be more inclined to do:
>
> typedef unsigned int vm_flags_t[2];

No, that's entirely invalid.

Never *ever* use arrays in C for type safety. Arrays are not type
safe. They can't be assigned sanely, and they silently become pointers
(which also aren't type-safe, since they end up converting silently to
'void *').

If you want to use the type system to enforce things, and you don't
want to rely on sparse, you absolutely have to use a struct (or union)
type.

So something like

   typedef struct { u64 val; } vm_flags_t;

would be an option.

              Linus