#ifdef very expensive debug check in page fault path
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>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memory.c b/mm/memory.c
index 4bf0b6d..6dd1cd8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -392,6 +392,7 @@
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 @@
print_bad_pte(vma, pte, addr);
return NULL;
}
+#endif
/*
* NOTE! We still have PageReserved() pages in the page