[v2 PATCH] mm: make CONFIG_DEFERRED_STRUCT_PAGE_INIT depends on !FLATMEM explicitly

[v2 PATCH] mm: make CONFIG_DEFERRED_STRUCT_PAGE_INIT depends on !FLATMEM explicitly

[Yang Shi]

Per the suggestion from Michal Hocko [1], DEFERRED_STRUCT_PAGE_INIT requires
some ordering wrt other initialization operations, e.g. page_ext_init has to
happen after the whole memmap is initialized properly. For SPARSEMEM this
requires to wait for page_alloc_init_late. Other memory models (e.g. flatmem)
might have different initialization layouts (page_ext_init_flatmem). Currently
DEFERRED_STRUCT_PAGE_INIT depends on MEMORY_HOTPLUG which in turn
	depends on SPARSEMEM || X86_64_ACPI_NUMA
	depends on ARCH_ENABLE_MEMORY_HOTPLUG

and X86_64_ACPI_NUMA depends on NUMA which in turn disable FLATMEM
memory model:
config ARCH_FLATMEM_ENABLE
	def_bool y
	depends on X86_32 && !NUMA

so FLATMEM is ruled out via dependency maze. Be explicit and disable
FLATMEM for DEFERRED_STRUCT_PAGE_INIT so that we do not reintroduce
subtle initialization bugs

[1] http://lkml.kernel.org/r/20160523073157.GD2278@dhcp22.suse.cz

CC: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Yang Shi <yang.shi@linaro.org>
---
v2:
 Adopted Michal's comments for the commit log

 mm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index 2664c11..22fa818 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -649,6 +649,7 @@ config DEFERRED_STRUCT_PAGE_INIT
 	default n
 	depends on ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
 	depends on MEMORY_HOTPLUG
+	depends on !FLATMEM
 	help
 	  Ordinarily all struct pages are initialised during early boot in a
 	  single thread. On very large machines this can take a considerable
-- 
2.0.2

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