linux-kernel - Re: [PATCH] [v2] page flags: prioritize kasan bits over last-cpuid

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Message-ID: <CAK8P3a1CAKecyinhzG9Mc7UzZ9U15o6nacbcfSvb4EBSaWvCTw@mail.gmail.com>
Date:   Tue, 18 Jun 2019 17:30:55 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Will Deacon <will.deacon@....com>,
        Christoph Lameter <cl@...ux.com>,
        Mark Rutland <mark.rutland@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [v2] page flags: prioritize kasan bits over last-cpuid

On Tue, Jun 18, 2019 at 4:30 PM Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
> On 6/18/19 12:53 PM, Arnd Bergmann wrote:
> > ARM64 randdconfig builds regularly run into a build error, especially
> > when NUMA_BALANCING and SPARSEMEM are enabled but not SPARSEMEM_VMEMMAP:
> >
> >  #error "KASAN: not enough bits in page flags for tag"
> >
> > The last-cpuid bits are already contitional on the available space,
> > so the result of the calculation is a bit random on whether they
> > were already left out or not.
> >
> > Adding the kasan tag bits before last-cpuid makes it much more likely
> > to end up with a successful build here, and should be reliable for
> > randconfig at least, as long as that does not randomize NR_CPUS
> > or NODES_SHIFT but uses the defaults.
> >
> > In order for the modified check to not trigger in the x86 vdso32 code
> > where all constants are wrong (building with -m32), enclose all the
> > definitions with an #ifdef.
> >
>
> Why not keep "#error "KASAN: not enough bits in page flags for tag"" under "#ifdef CONFIG_KASAN_SW_TAGS" ?

I think I had meant the #error to leave out the mention of KASAN, as there
might be other reasons for using up all the bits, but then I did not change
it in the end.

Should I remove the "KASAN" word or add the #ifdef when resending?

     Arnd