[patch] #ifdef very expensive debug check in page fault path

[Carsten Otte <cotte@de.ibm.com>]

This patch puts #ifdef CONFIG_DEBUG_VM around a check in vm_normal_page
that verifies that a pfn is valid. This patch increases performance of
the page fault microbenchmark in lmbench by 13% and overall dbench
performance by 7% on s390x.  pfn_valid() is an expensive operation on
s390 that needs a high double digit amount of CPU cycles.
Nick Piggin suggested that pfn_valid() involves an array lookup on
systems with sparsemem, and therefore is an expensive operation there
too.
The check looks like a clear debug thing to me, it should never trigger
on regular kernels. And if a pte is created for an invalid pfn, we'll
find out once the memory gets accessed later on anyway. Please consider
inclusion of this patch into mm.

Signed-off-by: Carsten Otte <cotte@de.ibm.com>
--- 
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -392,6 +392,7 @@ struct page *vm_normal_page(struct vm_ar
 			return NULL;
 	}
 
+#ifdef CONFIG_DEBUG_VM
 	/*
 	 * Add some anal sanity checks for now. Eventually,
 	 * we should just do "return pfn_to_page(pfn)", but
@@ -402,6 +403,7 @@ struct page *vm_normal_page(struct vm_ar
 		print_bad_pte(vma, pte, addr);
 		return NULL;
 	}
+#endif
 
 	/*
 	 * NOTE! We still have PageReserved() pages in the page 


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