From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 6D0BD900086 for ; Mon, 18 Apr 2011 03:27:35 -0400 (EDT) Received: by wyf19 with SMTP id 19so4972436wyf.14 for ; Mon, 18 Apr 2011 00:27:30 -0700 (PDT) Subject: Re: [PATCH] mm: Check if PTE is already allocated during page fault From: raz ben yehuda In-Reply-To: <20110415150606.GP15707@random.random> References: <20110415101248.GB22688@suse.de> <20110415143916.GN15707@random.random> <20110415150606.GP15707@random.random> Content-Type: text/plain; charset="UTF-8" Date: Mon, 18 Apr 2011 10:21:13 +0300 Message-ID: <1303111273.3425.0.camel@raz.scalemp.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: Mel Gorman , akpm@linux-foundation.org, riel@redhat.com, kosaki.motohiro@jp.fujitsu.com, lkml , linux-mm@kvack.org, stable@kernel.org, shai@scalex86.com patch works great. thank you Andrea. On Fri, 2011-04-15 at 17:06 +0200, Andrea Arcangeli wrote: > On Fri, Apr 15, 2011 at 04:39:16PM +0200, Andrea Arcangeli wrote: > > On Fri, Apr 15, 2011 at 11:12:4A.M +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 ;). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org