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Subject: Re: [PATCH] [v2] page flags: prioritize kasan bits over last-cpuid
To:     Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>, kasan-dev@googlegroups.com,
        linux-mm@kvack.org, Andrey Konovalov <andreyknvl@google.com>,
        Will Deacon <will.deacon@arm.com>,
        Christoph Lameter <cl@linux.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
References: <20190618095347.3850490-1-arnd@arndb.de>
From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
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Date:   Tue, 18 Jun 2019 17:30:02 +0300
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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" ?