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

[Arnd Bergmann]

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.

Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
Submitted v1 in March and never followed up on the build regression,
which is fixed in this version.
---
 include/linux/page-flags-layout.h | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
index 1dda31825ec4..7d794a629822 100644
--- a/include/linux/page-flags-layout.h
+++ b/include/linux/page-flags-layout.h
@@ -32,6 +32,7 @@
 
 #endif /* CONFIG_SPARSEMEM */
 
+#ifndef BUILD_VDSO32_64
 /*
  * page->flags layout:
  *
@@ -76,21 +77,23 @@
 #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
@@ -104,4 +107,5 @@
 #define LAST_CPUPID_NOT_IN_PAGE_FLAGS
 #endif
 
+#endif
 #endif /* _LINUX_PAGE_FLAGS_LAYOUT */
-- 
2.20.0