From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: unit tests and get_user_pages_ptes_fast() Date: Tue, 05 Oct 2010 16:25:05 +0200 Message-ID: <4CAB3541.3080505@redhat.com> References: <4CA99FE0.7070709@redhat.com> <20101004134052.GQ26357@random.random> <20101004235953.GA1474@amt.cnet> <4CAAD59B.5020003@redhat.com> <20101005092217.GA15663@amt.cnet> <20101005141547.GV26357@random.random> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , KVM list To: Andrea Arcangeli Return-path: Received: from mx1.redhat.com ([209.132.183.28]:5004 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753689Ab0JEOZO (ORCPT ); Tue, 5 Oct 2010 10:25:14 -0400 Received: from int-mx08.intmail.prod.int.phx2.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.21]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id o95EPC9f000915 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Tue, 5 Oct 2010 10:25:13 -0400 In-Reply-To: <20101005141547.GV26357@random.random> Sender: kvm-owner@vger.kernel.org List-ID: On 10/05/2010 04:15 PM, Andrea Arcangeli wrote: > On Tue, Oct 05, 2010 at 06:22:17AM -0300, Marcelo Tosatti wrote: > > It'll not be so advantageous for ksm because there should be read-faults > > very rarely on that case. > > It'll also make all clean swapcache dirty for no good. > > > Will post. > > If we've to walk pagetables twice, why don't you do this: > > writable=1 > get_user_pages_fast(write=write_fault) > if (!write_fault) > writable = __get_user_pages_fast(write=1) > > That will solve the debugging knob and it'll solve ksm and it'll be > optimal for read swapins on exclusive clean swapcache too. But it means an extra vmexit in the following case: - read fault - page is present and writeable in the Linux page table which is very common. For this you need get_user_pages_ptes_fast(). -- error compiling committee.c: too many arguments to function