linux-kernel - Re: [RFC PATCH 4/8] mm: Separate fault info out of 'struct vm

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Message-ID: <20210115092313.GA13700@willie-the-truck>
Date:   Fri, 15 Jan 2021 09:23:13 +0000
From:   Will Deacon <will@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <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>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC PATCH 4/8] mm: Separate fault info out of 'struct vm_fault'

On Thu, Jan 14, 2021 at 01:11:12PM -0800, Linus Torvalds wrote:
> On Thu, Jan 14, 2021 at 11:41 AM Will Deacon <will@...nel.org> wrote:
> >
> > Sure enough, an arm64 defconfig builds perfectly alright with that change,
> > but it really shouldn't. I'm using clang 11.0.5, so I had another go with
> > GCC 9.2.1 and bang:
> 
> Ok, looks like a clang bug, but a reasonably benign one.
> 
> As long as we have sufficient coverage with gcc, we'll get error
> reporting in a timely manner for any new incorrect assignments, so I
> think we can do that constant anonymous struct even if it does mean
> that clang might let some bad cases through (I personally use gcc for
> build testing, and then clang for building my boot kernels, so I'd
> catch anything x86-64 allmodconfig in my build tests).
> 
> And keeping it unnamed it would avoid a lot of noisy churn..

Hmm. The feedback on the clang bug suggests that GCC is the one in the
wrong here (although the argument is based on C11 and I haven't trawled
through the standards to see how this has evolved):

https://bugs.llvm.org/show_bug.cgi?id=48755#c1

There is at least some sympathy to generating a warning, so that might
be good enough. Otherwise, I suppose we can explicitly mark the fields
as 'const' but I won't jump to that immediately.

Will