Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753034Ab1FOR5M (ORCPT ); Wed, 15 Jun 2011 13:57:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42209 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751725Ab1FOR5J (ORCPT ); Wed, 15 Jun 2011 13:57:09 -0400 Date: Wed, 15 Jun 2011 19:54:55 +0200 From: Oleg Nesterov To: Srikar Dronamraju Cc: Peter Zijlstra , Ingo Molnar , Steven Rostedt , Linux-mm , Arnaldo Carvalho de Melo , Linus Torvalds , Hugh Dickins , Christoph Hellwig , Andi Kleen , Thomas Gleixner , Jonathan Corbet , Andrew Morton , Jim Keniston , Roland McGrath , Ananth N Mavinakayanahalli , LKML Subject: Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement. Message-ID: <20110615175455.GB12652@redhat.com> References: <20110607125804.28590.92092.sendpatchset@localhost6.localdomain6> <20110607125835.28590.25476.sendpatchset@localhost6.localdomain6> <20110613170020.GA27137@redhat.com> <20110614123530.GC4952@linux.vnet.ibm.com> <20110614142023.GA5139@redhat.com> <20110615085515.GE4952@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110615085515.GE4952@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2791 Lines: 74 On 06/15, Srikar Dronamraju wrote: > > > > > > + > > > > > + /* Read the page with vaddr into memory */ > > > > > + ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 1, 1, &old_page, &vma); > > > > > > > > Sorry if this was already discussed... But why we are using FOLL_WRITE here? > > > > We are not going to write into this page, and this provokes the unnecessary > > > > cow, no? > > > > > > Yes, We are not going to write to the page returned by get_user_pages > > > but a copy of that page. > > > > Yes I see. But the page returned by get_user_pages(write => 1) is already > > a cow'ed copy (this mapping should be read-only). > > > > > The idea was if we cow the page then we dont > > > need to cow it at the replace_page time > > > > Yes, replace_page() shouldn't cow. > > > > > and since get_user_pages knows > > > the right way to cow the page, we dont have to write another routine to > > > cow the page. > > > > Confused. write_opcode() allocs another page and does memcpy. This is > > correct, but I don't understand the first cow. > > > > we decided on get_user_pages(FOLL_WRITE|FOLL_FORCE) based on discussions > in these threads https://lkml.org/lkml/2010/4/23/327 and > https://lkml.org/lkml/2010/5/12/119 Failed to Connect. > Summary of those two sub-threads as I understand was to have > get_user_pages do the "real" cow for us. > > If I understand correctly, your concern is on the extra overhead added > by the get_user_pages. No. My main concern is that I do not understand why do we need an extra cow. This is fine, I am not vm expert. But I think it is not fine that you can't explain why your code needs it ;) What this 'get_user_pages do the "real" cow for us' actually means? It does not do cow for us, __replace_page() does the actual/final cow. It re-installs the modified copy of the page returned by get_user_pages() at the same pte. > > Probably I missed something... but could you please explain why we can't > > > > - ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 1, 1, &old_page, &vma); > > + ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 0, 0, &old_page, &vma); > > > > ? > > I tried the code with this change and it works for regular cases. > I am not sure if it affects cases where programs do mprotect Hmm... How can mprotect make a difference? This mapping should be read only, and we are not going to do pte_mkwrite. > So I am okay to not force cow through get_user_pages. I am okay either way ;) But, imho, if we use FOLL_WRITE|FOLL_FORCE then it would be nice to document why it this needed. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/