Commit 9723198c authored Jan 17, 2008 by Carsten Otte Committed by Linus Torvalds Jan 17, 2008

#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>

parent 6b2d2cec

mm/memory.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -392,6 +392,7 @@ struct page vm_normal_page(struct vm_area_struct vma, unsigned long addr, pte_
		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_area_struct vma, unsigned long addr, pte_
		print_bad_pte(vma, pte, addr);
		return NULL;
		}
		#endif

		/*
		* NOTE! We still have PageReserved() pages in the page