[PATCH] x86/mm: Fix phys_to_target_node() export

[Dan Williams <dan.j.williams@xxxxxxxxx>]

The core-mm has a default __weak implementation of phys_to_target_node()
when the architecture does not override it. That symbol is exported
for modules. However, while the export in mm/memory_hotplug.c exported
the symbol in the configuration cases of:

	CONFIG_NUMA_KEEP_MEMINFO=y
	CONFIG_MEMORY_HOTPLUG=y

...and:

	CONFIG_NUMA_KEEP_MEMINFO=n
	CONFIG_MEMORY_HOTPLUG=y

...it failed to export the symbol in the case of:

	CONFIG_NUMA_KEEP_MEMINFO=y
	CONFIG_MEMORY_HOTPLUG=n

Always export the symbol from the CONFIG_NUMA_KEEP_MEMINFO section of
arch/x86/mm/numa.c, and teach mm/memory_hotplug.c to optionally export
in case arch/x86/mm/numa.c has already performed the export.

The dependency on NUMA_KEEP_MEMINFO for DEV_DAX_HMEM_DEVICES is invalid
now that the symbol is properly exported in all combinations of
CONFIG_NUMA_KEEP_MEMINFO and CONFIG_MEMORY_HOTPLUG. Note that in the
CONFIG_NUMA=n case no export is needed since their is a dummy static
inline implementation of phys_to_target_node() in that case.

Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Reported-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Fixes: a035b6bf863e ("mm/memory_hotplug: introduce default phys_to_target_node() implementation")
Cc: Joao Martins <joao.m.martins@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/mm/numa.c  |    1 +
 drivers/dax/Kconfig |    1 -
 mm/memory_hotplug.c |    5 +++++
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
index 44148691d78b..e025947f19e0 100644
--- a/arch/x86/mm/numa.c
+++ b/arch/x86/mm/numa.c
@@ -938,6 +938,7 @@ int phys_to_target_node(phys_addr_t start)
 
 	return meminfo_to_nid(&numa_reserved_meminfo, start);
 }
+EXPORT_SYMBOL_GPL(phys_to_target_node);
 
 int memory_add_physaddr_to_nid(u64 start)
 {
diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index 567428e10b7b..d2834c2cfa10 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -50,7 +50,6 @@ config DEV_DAX_HMEM
 	  Say M if unsure.
 
 config DEV_DAX_HMEM_DEVICES
-	depends on NUMA_KEEP_MEMINFO # for phys_to_target_node()
 	depends on DEV_DAX_HMEM && DAX=y
 	def_bool y
 
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index b44d4c7ba73b..ed326b489674 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -365,9 +365,14 @@ int __weak phys_to_target_node(u64 start)
 			start);
 	return 0;
 }
+
+/* If the arch did not export a strong symbol, export the weak one. */
+#ifndef CONFIG_NUMA_KEEP_MEMINFO
 EXPORT_SYMBOL_GPL(phys_to_target_node);
 #endif
 
+#endif
+
 /* find the smallest valid pfn in the range [start_pfn, end_pfn) */
 static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
 				     unsigned long start_pfn,