From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757223Ab0GIPMR (ORCPT ); Fri, 9 Jul 2010 11:12:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63056 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751332Ab0GIPMQ (ORCPT ); Fri, 9 Jul 2010 11:12:16 -0400 Date: Fri, 9 Jul 2010 17:12:11 +0200 From: Andrea Arcangeli To: Jeremy Fitzhardinge Cc: Stefano Stabellini , Linux Kernel Mailing List Subject: Re: mmu notifier calls in apply_to_page_range() Message-ID: <20100709151211.GE13493@random.random> References: <4C373AEC.6000502@goop.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4C373AEC.6000502@goop.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 09, 2010 at 08:06:20AM -0700, Jeremy Fitzhardinge wrote: > I just noticed that the original mmu notifier change (cddb8a5c14a) adds > calls to mmu_notifier_invalidate_range_start/end to > apply_to_page_range(). This doesn't seem correct to me, since > apply_to_page_range can perform arbitrary operations to the range of > pages, not just invalidation of the pages. It seems to me that the > appropriate mmu notifiers should be called either around the call to > apply_to_page_range(), or from within the callback function. > > Andrea, what's the rationale for mmu_notifier_invalidate_range_start/end > here? As long as the secondary mappings are teardown in range_start and allowed to be established again only after range_end, all modifications will be picked up by the secondary mmu. Imagine secondary mmu like a tlb, that you only invalidate, then it'll be refilled later (after range_end). The exception is set_pte_at_notify that is called by ksm to establish a readonly secondary pte in KVM, KVM only calls get_user_pages(write=1) (never write=0 even for reads) so until that is optimized set_pte_at_notify allows guest to access readonly data without breaking the cow. set_pte_at_notify invokes a change_pte method, if not implemented it'll just fallback to the invalidate_page method that is backwards compatible, so no mmu notifier user is required to call change_pte (especially if the secondary page fault - kind of secondary tlb-miss software handler invokes get_user_pages with write=0 for reads, ->change_pte can only eliminate one minor fault so no big deal).