From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754524Ab1DOPGT (ORCPT ); Fri, 15 Apr 2011 11:06:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:12105 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754105Ab1DOPGS (ORCPT ); Fri, 15 Apr 2011 11:06:18 -0400 Date: Fri, 15 Apr 2011 17:06:06 +0200 From: Andrea Arcangeli To: Mel Gorman Cc: akpm@linux-foundation.org, raz ben yehuda , riel@redhat.com, kosaki.motohiro@jp.fujitsu.com, lkml , linux-mm@kvack.org, stable@kernel.org Subject: Re: [PATCH] mm: Check if PTE is already allocated during page fault Message-ID: <20110415150606.GP15707@random.random> References: <20110415101248.GB22688@suse.de> <20110415143916.GN15707@random.random> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110415143916.GN15707@random.random> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 15, 2011 at 04:39:16PM +0200, Andrea Arcangeli wrote: > On Fri, Apr 15, 2011 at 11:12:48AM +0100, Mel Gorman wrote: > > diff --git a/mm/memory.c b/mm/memory.c > > index 5823698..1659574 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -3322,7 +3322,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, > > * run pte_offset_map on the pmd, if an huge pmd could > > * materialize from under us from a different thread. > > */ > > - if (unlikely(__pte_alloc(mm, vma, pmd, address))) > > + if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address)) I started hacking on this and I noticed it'd be better to extend the unlikely through the end. At first review I didn't notice the parenthesis closure stops after pte_none and __pte_alloc is now uncovered. I'd prefer this: if (unlikely(pmd_none(*pmd) && __pte_alloc(mm, vma, pmd, address))) I mean the real unlikely thing is that we return VM_FAULT_OOM, if we end up calling __pte_alloc or not, depends on the app. Generally it sounds more frequent that the pte is not none, so it's not wrong, but it's even less likely that __pte_alloc fails so that can be taken into account too, and __pte_alloc runs still quite frequently. So either above or: if (unlikely(pmd_none(*pmd)) && unlikely(__pte_alloc(mm, vma, pmd, address))) I generally prefer unlikely only when it's 100% sure thing it's less likely (like the VM_FAULT_OOM), so the first version I guess it's enough (I'm afraid unlikely for pte_none too, may make gcc generate a far away jump possibly going out of l1 icache for a case that is only 512 times less likely at best). My point is that it's certainly hugely more unlikely that __pte_alloc fails than the pte is none. This is a real nitpick though ;).