x86/mm: Simplify enabling direct_gbpages

direct_gbpages can be force enabled as an early parameter
but not really have taken effect when DEBUG_PAGEALLOC
or KMEMCHECK is enabled. You can also enable direct_gbpages
right now if you have an x86_64 architecture but your CPU
doesn't really have support for this feature. In both cases
PG_LEVEL_1G won't actually be enabled but direct_gbpages is used
in other areas under the assumptions that PG_LEVEL_1G
was set. Fix this by putting together all requirements
which make this feature sensible to enable under, and only
enable both finally flipping on PG_LEVEL_1G and leaving
PG_LEVEL_1G set when this is true.

We only enable this feature then to be possible on sensible
builds defined by the new ENABLE_DIRECT_GBPAGES. If the
CPU has support for it you can either enable this by using
the DIRECT_GBPAGES option or using the early kernel parameter.
If a platform had support for this you can always force disable
it as well.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: JBeulich@suse.com
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Toshi Kani <toshi.kani@hp.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: julia.lawall@lip6.fr
Link: http://lkml.kernel.org/r/1425518654-3403-3-git-send-email-mcgrof@do-not-panic.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 13 insertions, 5 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c2fb8a87dccb..4d06e1c8294a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1299,14 +1299,22 @@ config ARCH_DMA_ADDR_T_64BIT
 	def_bool y
 	depends on X86_64 || HIGHMEM64G
 
+config ENABLE_DIRECT_GBPAGES
+	def_bool y
+	depends on X86_64 && !DEBUG_PAGEALLOC && !KMEMCHECK
+
 config DIRECT_GBPAGES
 	bool "Enable 1GB pages for kernel pagetables" if EXPERT
 	default y
-	depends on X86_64
-	---help---
-	  Allow the kernel linear mapping to use 1GB pages on CPUs that
-	  support it. This can improve the kernel's performance a tiny bit by
-	  reducing TLB pressure. If in doubt, say "Y".
+	depends on ENABLE_DIRECT_GBPAGES
+	---help---
+	  Enable by default the kernel linear mapping to use 1GB pages on CPUs
+	  that support it. This can improve the kernel's performance a tiny bit
+	  by reducing TLB pressure. If in doubt, say "Y". If you've disabled
+	  option but your platform is capable of handling support for this
+	  you can use the gbpages kernel parameter. Likewise if you've enabled
+	  this but you'd like to force disable this option you can use the
+	  nogbpages kernel parameter.
 
 # Common NUMA Features
 config NUMA