[09/17] VFALLBACK: Debugging aid

[Christoph Lameter <clameter@sgi.com>]

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Virtual fallbacks are rare and thus subtle bugs may creep in if we do not
test the fallbacks. CONFIG_VFALLBACK_ALWAYS makes all GFP_VFALLBACK
allocations fall back to virtual mapping.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 lib/Kconfig.debug |   11 +++++++++++
 mm/page_alloc.c   |    9 +++++++++
 2 files changed, 20 insertions(+)

Index: linux-2.6/mm/page_alloc.c
===================================================================
--- linux-2.6.orig/mm/page_alloc.c	2007-09-18 19:19:34.000000000 -0700
+++ linux-2.6/mm/page_alloc.c	2007-09-18 20:16:26.000000000 -0700
@@ -1205,7 +1205,16 @@ zonelist_scan:
 					goto this_zone_full;
 			}
 		}
+#ifdef CONFIG_VFALLBACK_ALWAYS
+		if ((gfp_mask & __GFP_VFALLBACK) &&
+				system_state == SYSTEM_RUNNING)  {
+			struct page *vcompound_alloc(gfp_t, int,
+					struct zonelist *, unsigned long);
 
+			page = vcompound_alloc(gfp_mask, order,
+						zonelist, alloc_flags);
+		} else
+#endif
 		page = buffered_rmqueue(zonelist, zone, order, gfp_mask);
 		if (page)
 			break;
Index: linux-2.6/lib/Kconfig.debug
===================================================================
--- linux-2.6.orig/lib/Kconfig.debug	2007-09-18 19:19:28.000000000 -0700
+++ linux-2.6/lib/Kconfig.debug	2007-09-18 19:19:34.000000000 -0700
@@ -105,6 +105,17 @@ config DETECT_SOFTLOCKUP
 	   can be detected via the NMI-watchdog, on platforms that
 	   support it.)
 
+config VFALLBACK_ALWAYS
+	bool "Always fall back to Virtual Compound pages"
+	default y
+	help
+	  Virtual compound pages are only allocated if there is no linear
+	  memory available. They are a fallback and errors created by the
+	  use of virtual mappings instead of linear ones may not surface
+	  because of their infrequent use. This option makes every
+	  allocation that allows a fallback to a virtual mapping use
+	  the virtual mapping. May have a significant performance impact.
+
 config SCHED_DEBUG
 	bool "Collect scheduler debugging info"
 	depends on DEBUG_KERNEL && PROC_FS

--