From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932091Ab2FNSgt (ORCPT ); Thu, 14 Jun 2012 14:36:49 -0400 Received: from acsinet15.oracle.com ([141.146.126.227]:32484 "EHLO acsinet15.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755831Ab2FNSgr (ORCPT ); Thu, 14 Jun 2012 14:36:47 -0400 Date: Thu, 14 Jun 2012 14:29:13 -0400 From: Konrad Rzeszutek Wilk To: David Vrabel , rusty@rustcorp.com.au Cc: "xen-devel@lists.xensource.com" , "H. Peter Anvin" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 0/2] x86/mm: remove arch-specific PTE/PMD get-and-clear functions Message-ID: <20120614182913.GA21956@phenom.dumpdata.com> References: <1339582845-25659-1-git-send-email-david.vrabel@citrix.com> <20120613140404.GH5979@phenom.dumpdata.com> <4FD8AB07.7080004@citrix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4FD8AB07.7080004@citrix.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Source-IP: acsinet22.oracle.com [141.146.126.238] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 13, 2012 at 04:00:23PM +0100, David Vrabel wrote: > On 13/06/12 15:04, Konrad Rzeszutek Wilk wrote: > > On Wed, Jun 13, 2012 at 11:20:43AM +0100, David Vrabel wrote: > >> This series removes the x86-specific implementation of > >> ptep_get_and_clear() and pmdp_get_and_clear(). > >> > >> The principal reason for this is it allows Xen paravitualized guests > >> to batch the PTE clears which is a significant performance > >> optimization of munmap() and mremap() -- the number of entries into > >> the hypervisor is reduced by about a factor of about 30 (60 in 32-bit > >> guests) for munmap(). > >> > >> There may be minimal gains on native and KVM guests due to the removal > >> of the locked xchg. > > > > What about lguest? > > As I note in the description of patch 1: > > "There may be a performance regression with lguest guests as > an optimization for avoiding calling pte_update() when doing a full > teardown of an mm is removed." > > I don't know how much this performance regression would be or if the > performance of lguest guests is something people care about. > > We could have an x86-specific ptep_get_and_clear_full() which looks like: > > pte_t ptep_get_and_clear_full( > struct mm_struct *mm, unsigned long addr, pte_t *ptep, > int full) > { > pte_t pte = *ptep; > > pte_clear(mm, address, ptep); > if (!full) > pte_update(mm, addr, ptep); > > return pte; > } > > Which would have all the performance benefits of the proposed patch > without the performance regression with lguest. Lets rope Rusty in this since he is the maintainer of lguest. > > David > > >> > >> Removal of arch-specific functions where generic ones are suitable > >> seems to be a generally useful thing to me. > >> > >> The full reasoning for why this is safe is included in the commit > >> message of patch 1 but to summarize. The atomic get-and-clear does > >> not guarantee that the latest dirty/accessed bits are returned as TLB > >> as there is a still a window after the get-and-clear and before the > >> TLB flush that the bits may be updated on other processors. So, user > >> space applications accessing pages that are being unmapped or remapped > >> already have unpredictable behaviour. > >> > >> David