linux-kernel - Re: [PATCH v4 8/8] mm: Mark anonymous struct field of 'struct vm

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Date:   Thu, 21 Jan 2021 21:28:32 +0000
From:   Will Deacon <will@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Hugh Dickins <hughd@...gle.com>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH v4 8/8] mm: Mark anonymous struct field of 'struct
 vm_fault' as 'const'

On Thu, Jan 21, 2021 at 11:24:36AM -0800, Nick Desaulniers wrote:
> On Thu, Jan 21, 2021 at 5:11 AM Will Deacon <will@...nel.org> wrote:
> >
> > On Wed, Jan 20, 2021 at 11:02:06AM -0800, Linus Torvalds wrote:
> > > On Wed, Jan 20, 2021 at 10:27 AM Nick Desaulniers
> > > <ndesaulniers@...gle.com> wrote:
> > > >
> > > > Is there a difference between: [ const unnamed struct and individual const members ]
> > >
> > > Semantically? No.
> > >
> > > Syntactically the "group the const members together" is a lot cleaner,
> > > imho. Not just from a "just a single const" standpoint, but from a
> > > "code as documentation" standpoint.
> > >
> > > But I guess to avoid the clang issue, we could do the "mark individual
> > > fields" thing.
> >
> > I'd prefer to wait until the bug against LLVM has been resolved before we
> > try to work around anything. Although I couldn't find any other examples
> > like this in the kernel, requiring all of the member fields to be marked as
> > 'const' still feels pretty fragile to me; it's only a matter of time before
> > new non-const fields get added, at which point the temptation for developers
> > to remove 'const' from other fields when it gets in the way is pretty high.
> 
> What's to stop a new non-const field from getting added outside the
> const qualified anonymous struct?
> What's to stop someone from removing const from the anonymous struct?
> What's to stop a number of callers from manipulating the structure
> temporarily before restoring it when returning by casting away the
> const?
> 
> Code review.

Sure, but here we are cleaning up this stuff, so I think review only gets
you so far. To me:

	const struct {
		int	foo;
		long	bar;
	};

clearly says "don't modify fields of this struct", whereas:

	struct {
		const int	foo;
		const long	bar;
	};

says "don't modify foo or bar, but add whatever you like on the end" and
that's the slippery slope. So then we end up with the eye-sore of:

	const struct {
		const int	foo;
		const long	bar;
	};

and maybe that's the right answer, but I'm just saying we should wait for
clang to make up its mind first. It's not like this is a functional problem,
and there are enough GCC users around that we're not exactly in a hurry.

Will