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

[Andrey Ryabinin]

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" ?