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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 6 Jan 2023 14:06:51 -0800
Message-ID: <CAHk-=wj7jenrb6UNjv--xLC4hrjJDsCFxuaHw1e67a4ihVEmUw@mail.gmail.com>
Subject: Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings
To:     Matthew Wilcox <willy@infradead.org>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Yann Droneaud <ydroneaud@opteya.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, tglx@linutronix.de,
        linux-crypto@vger.kernel.org, linux-api@vger.kernel.org,
        x86@kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
        "Carlos O'Donell" <carlos@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Jann Horn <jannh@google.com>,
        Christian Brauner <brauner@kernel.org>, linux-mm@kvack.org
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On Fri, Jan 6, 2023 at 1:42 PM Matthew Wilcox <willy@infradead.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