From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757383Ab3KZXG1 (ORCPT ); Tue, 26 Nov 2013 18:06:27 -0500 Received: from mx1.redhat.com ([209.132.183.28]:28349 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753613Ab3KZXGY (ORCPT ); Tue, 26 Nov 2013 18:06:24 -0500 Date: Tue, 26 Nov 2013 17:31:48 -0200 From: Marcelo Tosatti To: Xiao Guangrong Cc: Gleb Natapov , avi.kivity@gmail.com, "pbonzini@redhat.com Bonzini" , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Eric Dumazet , Peter Zijlstra Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc Message-ID: <20131126193148.GA18071@amt.cnet> References: <1382534973-13197-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <1382534973-13197-8-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <20131122191429.GA13308@amt.cnet> <65EE805B-B5DB-4BD0-A057-E5FF78D96D67@linux.vnet.ibm.com> <5292EE2F.5090305@linux.vnet.ibm.com> <20131125181254.GB21858@amt.cnet> <529413C1.60302@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <529413C1.60302@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 26, 2013 at 11:21:37AM +0800, Xiao Guangrong wrote: > On 11/26/2013 02:12 AM, Marcelo Tosatti wrote: > > On Mon, Nov 25, 2013 at 02:29:03PM +0800, Xiao Guangrong wrote: > >>>> Also, there is no guarantee of termination (as long as sptes are > >>>> deleted with the correct timing). BTW, can't see any guarantee of > >>>> termination for rculist nulls either (a writer can race with a lockless > >>>> reader indefinately, restarting the lockless walk every time). > >>> > >>> Hmm, that can be avoided by checking dirty-bitmap before rewalk, > >>> that means, if the dirty-bitmap has been set during lockless write-protection, > >>> it�s unnecessary to write-protect its sptes. Your idea? > >> This idea is based on the fact that the number of rmap is limited by > >> RMAP_RECYCLE_THRESHOLD. So, in the case of adding new spte into rmap, > >> we can break the rewalk at once, in the case of deleting, we can only > >> rewalk RMAP_RECYCLE_THRESHOLD times. > > > > Please explain in more detail. > > Okay. > > My proposal is like this: > > pte_list_walk_lockless() > { > restart: > > + if (__test_bit(slot->arch.dirty_bitmap, gfn-index)) > + return; > > code-doing-lockless-walking; > ...... > } > > Before do lockless-walking, we check the dirty-bitmap first, if > it is set we can simply skip write-protection for the gfn, that > is the case that new spte is being added into rmap when we lockless > access the rmap. The dirty bit could be set after the check. > For the case of deleting spte from rmap, the number of entry is limited > by RMAP_RECYCLE_THRESHOLD, that is not endlessly. It can shrink and grow while lockless walk is performed.