thp: transparent hugepage config choice

Allow to choose between the always|madvise default for page faults and
khugepaged at config time.  madvise guarantees zero risk of higher memory
footprint for applications (applications using madvise(MADV_HUGEPAGE)
won't risk to use any more memory by backing their virtual regions with
hugepages).

Initially set the default to N and don't depend on EMBEDDED.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/Kconfig

@@ -303,9 +303,8 @@ config NOMMU_INITIAL_TRIM_EXCESS
 	  See Documentation/nommu-mmap.txt for more information.
 
 config TRANSPARENT_HUGEPAGE
-	bool "Transparent Hugepage Support" if EMBEDDED
+	bool "Transparent Hugepage Support"
 	depends on X86 && MMU
-	default y
 	help
 	  Transparent Hugepages allows the kernel to use huge pages and
 	  huge tlb transparently to the applications whenever possible.
@@ -316,6 +315,30 @@ config TRANSPARENT_HUGEPAGE
 
 	  If memory constrained on embedded, you may want to say N.
 
+choice
+	prompt "Transparent Hugepage Support sysfs defaults"
+	depends on TRANSPARENT_HUGEPAGE
+	default TRANSPARENT_HUGEPAGE_ALWAYS
+	help
+	  Selects the sysfs defaults for Transparent Hugepage Support.
+
+	config TRANSPARENT_HUGEPAGE_ALWAYS
+		bool "always"
+	help
+	  Enabling Transparent Hugepage always, can increase the
+	  memory footprint of applications without a guaranteed
+	  benefit but it will work automatically for all applications.
+
+	config TRANSPARENT_HUGEPAGE_MADVISE
+		bool "madvise"
+	help
+	  Enabling Transparent Hugepage madvise, will only provide a
+	  performance improvement benefit to the applications using
+	  madvise(MADV_HUGEPAGE) but it won't risk to increase the
+	  memory footprint of applications without a guaranteed
+	  benefit.
+endchoice
+
 #
 # UP and nommu archs use km based percpu allocator
 #

mm/huge_memory.c

@@ -27,7 +27,12 @@
  * allocations.
  */
 unsigned long transparent_hugepage_flags __read_mostly =
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS
 	(1<<TRANSPARENT_HUGEPAGE_FLAG)|
+#endif
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
+	(1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG)|
+#endif
 	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_FLAG)|
 	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);