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

[Andrey Konovalov]

On Thu, Mar 7, 2019 at 8:51 AM Arnd Bergmann <arnd@xxxxxxxx> 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.
>
> Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  include/linux/page-flags-layout.h | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
> index 1dda31825ec4..9bc0751e68b2 100644
> --- a/include/linux/page-flags-layout.h
> +++ b/include/linux/page-flags-layout.h
> @@ -76,21 +76,23 @@
>  #define LAST_CPUPID_SHIFT 0
>  #endif
>
> -#if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
> +#ifdef CONFIG_KASAN_SW_TAGS
> +#define KASAN_TAG_WIDTH 8
> +#else
> +#define KASAN_TAG_WIDTH 0
> +#endif
> +
> +#if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT+KASAN_TAG_WIDTH \
> +       <= BITS_PER_LONG - NR_PAGEFLAGS
>  #define LAST_CPUPID_WIDTH LAST_CPUPID_SHIFT
>  #else
>  #define LAST_CPUPID_WIDTH 0
>  #endif
>
> -#ifdef CONFIG_KASAN_SW_TAGS
> -#define KASAN_TAG_WIDTH 8
>  #if SECTIONS_WIDTH+NODES_WIDTH+ZONES_WIDTH+LAST_CPUPID_WIDTH+KASAN_TAG_WIDTH \
>         > BITS_PER_LONG - NR_PAGEFLAGS
>  #error "KASAN: not enough bits in page flags for tag"
>  #endif
> -#else
> -#define KASAN_TAG_WIDTH 0
> -#endif
>
>  /*
>   * We are going to use the flags for the page to node mapping if its in
> --
> 2.20.0
>

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

Thanks!