mm: Do not enable PG_arch_2 for all 64-bit architectures

Commit 4beba948 ("mm: Add PG_arch_2 page flag") introduced a new
page flag for all 64-bit architectures. However, even if an architecture
is 64-bit, it may still have limited spare bits in the 'flags' member of
'struct page'. This may happen if an architecture enables SPARSEMEM
without SPARSEMEM_VMEMMAP as is the case with the newly added loongarch.
This architecture port needs 19 more bits for the sparsemem section
information and, while it is currently fine with PG_arch_2, adding any
more PG_arch_* flags will trigger build-time warnings.

Add a new CONFIG_ARCH_USES_PG_ARCH_X option which can be selected by
architectures that need more PG_arch_* flags beyond PG_arch_1. Select it
on arm64.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
[pcc@google.com: fix build with CONFIG_ARM64_MTE disabled]
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Price <steven.price@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221104011041.290951-2-pcc@google.com

arch/arm64/Kconfig

@@ -1965,6 +1965,7 @@ config ARM64_MTE
 	depends on ARM64_PAN
 	select ARCH_HAS_SUBPAGE_FAULTS
 	select ARCH_USES_HIGH_VMA_FLAGS
+	select ARCH_USES_PG_ARCH_X
 	help
 	  Memory Tagging (part of the ARMv8.5 Extensions) provides
 	  architectural support for run-time, always-on detection of

fs/proc/page.c

@@ -219,7 +219,7 @@ u64 stable_page_flags(struct page *page)
 	u |= kpf_copy_bit(k, KPF_PRIVATE_2,	PG_private_2);
 	u |= kpf_copy_bit(k, KPF_OWNER_PRIVATE,	PG_owner_priv_1);
 	u |= kpf_copy_bit(k, KPF_ARCH,		PG_arch_1);
-#ifdef CONFIG_64BIT
+#ifdef CONFIG_ARCH_USES_PG_ARCH_X
 	u |= kpf_copy_bit(k, KPF_ARCH_2,	PG_arch_2);
 #endif
 

include/linux/page-flags.h

@@ -132,7 +132,7 @@ enum pageflags {
 	PG_young,
 	PG_idle,
 #endif
-#ifdef CONFIG_64BIT
+#ifdef CONFIG_ARCH_USES_PG_ARCH_X
 	PG_arch_2,
 #endif
 #ifdef CONFIG_KASAN_HW_TAGS

include/trace/events/mmflags.h

@@ -91,10 +91,10 @@
 #define IF_HAVE_PG_IDLE(flag,string)
 #endif
 
-#ifdef CONFIG_64BIT
-#define IF_HAVE_PG_ARCH_2(flag,string) ,{1UL << flag, string}
+#ifdef CONFIG_ARCH_USES_PG_ARCH_X
+#define IF_HAVE_PG_ARCH_X(flag,string) ,{1UL << flag, string}
 #else
-#define IF_HAVE_PG_ARCH_2(flag,string)
+#define IF_HAVE_PG_ARCH_X(flag,string)
 #endif
 
 #ifdef CONFIG_KASAN_HW_TAGS
@@ -130,7 +130,7 @@ IF_HAVE_PG_UNCACHED(PG_uncached,	"uncached"	)		\
 IF_HAVE_PG_HWPOISON(PG_hwpoison,	"hwpoison"	)		\
 IF_HAVE_PG_IDLE(PG_young,		"young"		)		\
 IF_HAVE_PG_IDLE(PG_idle,		"idle"		)		\
-IF_HAVE_PG_ARCH_2(PG_arch_2,		"arch_2"	)		\
+IF_HAVE_PG_ARCH_X(PG_arch_2,		"arch_2"	)		\
 IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
 
 #define show_page_flags(flags)						\

mm/Kconfig

@@ -1005,6 +1005,14 @@ config ARCH_USES_HIGH_VMA_FLAGS
 config ARCH_HAS_PKEYS
 	bool
 
+config ARCH_USES_PG_ARCH_X
+	bool
+	help
+	  Enable the definition of PG_arch_x page flags with x > 1. Only
+	  suitable for 64-bit architectures with CONFIG_FLATMEM or
+	  CONFIG_SPARSEMEM_VMEMMAP enabled, otherwise there may not be
+	  enough room for additional bits in page->flags.
+
 config VM_EVENT_COUNTERS
 	default y
 	bool "Enable VM event counters for /proc/vmstat" if EXPERT

mm/huge_memory.c

@@ -2444,7 +2444,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
 			 (1L << PG_workingset) |
 			 (1L << PG_locked) |
 			 (1L << PG_unevictable) |
-#ifdef CONFIG_64BIT
+#ifdef CONFIG_ARCH_USES_PG_ARCH_X
 			 (1L << PG_arch_2) |
 #endif
 			 (1L << PG_dirty) |