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

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Date:   Tue, 18 Jun 2019 19:28:10 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Arnd Bergmann <arnd@...db.de>
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 6/18/19 6:30 PM, Arnd Bergmann wrote:
> 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?

It seems like changing the error message is a better choice.
Don't forget to remove "for tag" as well.