From: Carsten Otte Date: Thu, 17 Jan 2008 23:21:17 +0000 (-0800) Subject: #ifdef very expensive debug check in page fault path X-Git-Tag: v2.6.24~52 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9723198c219f3546982cb469e5aed26e68399055;p=linux-2.6 #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 Acked-by: Nick Piggin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/memory.c b/mm/memory.c index 4bf0b6d0eb..6dd1cd88bf 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -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