[PATCH] arm64: fix vmemmap BUILD_BUG_ON() triggering on !vmemmap setups

[Johannes Weiner]

Arnd reports the following arm64 randconfig build error with the PSI
patches that add another page flag:

  /git/arm-soc/arch/arm64/mm/init.c: In function 'mem_init':
  /git/arm-soc/include/linux/compiler.h:357:38: error: call to
  '__compiletime_assert_618' declared with attribute error: BUILD_BUG_ON
  failed: sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT)

The additional page flag causes other information stored in
page->flags to get bumped into their own struct page member:

  #if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT <=
  BITS_PER_LONG - NR_PAGEFLAGS
  #define LAST_CPUPID_WIDTH LAST_CPUPID_SHIFT
  #else
  #define LAST_CPUPID_WIDTH 0
  #endif

  #if defined(CONFIG_NUMA_BALANCING) && LAST_CPUPID_WIDTH == 0
  #define LAST_CPUPID_NOT_IN_PAGE_FLAGS
  #endif

which in turn causes the struct page size to exceed the size set in
STRUCT_PAGE_MAX_SHIFT. This value is an an estimate used to size the
VMEMMAP page array according to address space and struct page size.

However, the check is performed - and triggers here - on a !VMEMMAP
config, which consumes an additional 22 page bits for the sparse
section id. When VMEMMAP is enabled, those bits are returned, cpupid
doesn't need its own member, and the page passes the VMEMMAP check.

Restrict that check to the situation it was meant to check: that we
are sizing the VMEMMAP page array correctly.

Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 arch/arm64/mm/init.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 1b18b4722420..72c9b6778b0a 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -611,11 +611,13 @@ void __init mem_init(void)
 	BUILD_BUG_ON(TASK_SIZE_32			> TASK_SIZE_64);
 #endif
 
+#ifndef CONFIG_SPARSEMEM_VMEMMAP
 	/*
 	 * Make sure we chose the upper bound of sizeof(struct page)
-	 * correctly.
+	 * correctly when sizing the VMEMMAP array.
 	 */
 	BUILD_BUG_ON(sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT));
+#endif
 
 	if (PAGE_SIZE >= 16384 && get_num_physpages() <= 128) {
 		extern int sysctl_overcommit_memory;
-- 
2.18.0