From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=W9oF=D5=kvack.org=owner-linux-mm@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
X-Spam-Level: 
X-Spam-Status: No, score=-5.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH,
	DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1
	autolearn=no autolearn_force=no version=3.4.0
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id EC6EFC5517A
	for <linux-mm@archiver.kernel.org>; Thu, 22 Oct 2020 19:58:20 +0000 (UTC)
Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17])
	by mail.kernel.org (Postfix) with ESMTP id 2E20D2417D
	for <linux-mm@archiver.kernel.org>; Thu, 22 Oct 2020 19:58:19 +0000 (UTC)
Authentication-Results: mail.kernel.org;
	dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="bs3xt+dz"
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2E20D2417D
Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=nvidia.com
Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org
Received: by kanga.kvack.org (Postfix)
	id 5D67C6B005D; Thu, 22 Oct 2020 15:58:19 -0400 (EDT)
Received: by kanga.kvack.org (Postfix, from userid 40)
	id 560BF6B0062; Thu, 22 Oct 2020 15:58:19 -0400 (EDT)
X-Delivered-To: int-list-linux-mm@kvack.org
Received: by kanga.kvack.org (Postfix, from userid 63042)
	id 428296B0068; Thu, 22 Oct 2020 15:58:19 -0400 (EDT)
X-Delivered-To: linux-mm@kvack.org
Received: from forelay.hostedemail.com (smtprelay0079.hostedemail.com [216.40.44.79])
	by kanga.kvack.org (Postfix) with ESMTP id 12EEE6B005D
	for <linux-mm@kvack.org>; Thu, 22 Oct 2020 15:58:19 -0400 (EDT)
Received: from smtpin10.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251])
	by forelay04.hostedemail.com (Postfix) with ESMTP id ACD6E1EE6
	for <linux-mm@kvack.org>; Thu, 22 Oct 2020 19:58:18 +0000 (UTC)
X-FDA: 77400623076.10.north72_2b12a0d27253
Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251])
	by smtpin10.hostedemail.com (Postfix) with ESMTP id 6F81916A4B1
	for <linux-mm@kvack.org>; Thu, 22 Oct 2020 19:58:18 +0000 (UTC)
X-HE-Tag: north72_2b12a0d27253
X-Filterd-Recvd-Size: 5124
Received: from hqnvemgate24.nvidia.com (hqnvemgate24.nvidia.com [216.228.121.143])
	by imf32.hostedemail.com (Postfix) with ESMTP
	for <linux-mm@kvack.org>; Thu, 22 Oct 2020 19:58:17 +0000 (UTC)
Received: from hqmail.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate24.nvidia.com (using TLS: TLSv1.2, AES256-SHA)
	id <B5f91e3fa0001>; Thu, 22 Oct 2020 12:56:42 -0700
Received: from [10.2.54.36] (10.124.1.5) by HQMAIL107.nvidia.com
 (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Thu, 22 Oct
 2020 19:58:14 +0000
Subject: Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to
 access guest memory
To: Matthew Wilcox <willy@infradead.org>
CC: "Kirill A. Shutemov" <kirill@shutemov.name>, Dave Hansen
	<dave.hansen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>, "Peter
 Zijlstra" <peterz@infradead.org>, Paolo Bonzini <pbonzini@redhat.com>, "Sean
 Christopherson" <sean.j.christopherson@intel.com>, Vitaly Kuznetsov
	<vkuznets@redhat.com>, Wanpeng Li <wanpengli@tencent.com>, Jim Mattson
	<jmattson@google.com>, Joerg Roedel <joro@8bytes.org>, David Rientjes
	<rientjes@google.com>, Andrea Arcangeli <aarcange@redhat.com>, Kees Cook
	<keescook@chromium.org>, Will Drewry <wad@chromium.org>, "Edgecombe, Rick P"
	<rick.p.edgecombe@intel.com>, "Kleen, Andi" <andi.kleen@intel.com>, "Liran
 Alon" <liran.alon@oracle.com>, Mike Rapoport <rppt@kernel.org>,
	<x86@kernel.org>, <kvm@vger.kernel.org>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>, "Kirill A. Shutemov"
	<kirill.shutemov@linux.intel.com>
References: <20201020061859.18385-1-kirill.shutemov@linux.intel.com>
 <20201020061859.18385-9-kirill.shutemov@linux.intel.com>
 <c8b0405f-14ed-a1bb-3a91-586a30bdf39b@nvidia.com>
 <20201022114946.GR20115@casper.infradead.org>
From: John Hubbard <jhubbard@nvidia.com>
Message-ID: <30ce6691-fd70-76a2-8b61-86d207c88713@nvidia.com>
Date: Thu, 22 Oct 2020 12:58:14 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.12.0
MIME-Version: 1.0
In-Reply-To: <20201022114946.GR20115@casper.infradead.org>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
X-Originating-IP: [10.124.1.5]
X-ClientProxiedBy: HQMAIL105.nvidia.com (172.20.187.12) To
 HQMAIL107.nvidia.com (172.20.187.13)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1;
	t=1603396602; bh=SDDfdPapwaAGPN6G5NGvPgR723v4Q3ngUYge890SCck=;
	h=Subject:To:CC:References:From:Message-ID:Date:User-Agent:
	 MIME-Version:In-Reply-To:Content-Type:Content-Language:
	 Content-Transfer-Encoding:X-Originating-IP:X-ClientProxiedBy;
	b=bs3xt+dzMIEc1iPoqfF/PiRE0RAm4lrjLAAOpczF8TnctMMyV4Q6U4C16Gw/BxU00
	 7D0s51rhGaNO1lWwWg1UNN5wmXuNlG2QNHfTbbZxkpe8Yd8/FwKxpdss1g1AZ4bReG
	 /Lg5LHvimXwF5n3bsNikgJ8lN6fbTTyz4NqL2mcGXn41IoQ2lmRaqx1bp+/171WoDZ
	 b+F+ihCsmrVAtPDqwRPIj0cNvYeQ1uMbGVK00N+kUXU/mdTBBViG6uWtt+N5kwf53P
	 xVAASH+PC4G9wiuXgn3Rf/c0FTJFfb0x1cK1SXN38pgZeEQwQth9jFXkRQj3NJUxOR
	 nMrdMYnlmyBbg==
X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4
Sender: owner-linux-mm@kvack.org
Precedence: bulk
X-Loop: owner-majordomo@kvack.org
List-ID: <linux-mm.kvack.org>

On 10/22/20 4:49 AM, Matthew Wilcox wrote:
> On Tue, Oct 20, 2020 at 01:25:59AM -0700, John Hubbard wrote:
>> Should copy_to_guest() use pin_user_pages_unlocked() instead of gup_unlocked?
>> We wrote a  "Case 5" in Documentation/core-api/pin_user_pages.rst, just for this
>> situation, I think:
>>
>>
>> CASE 5: Pinning in order to write to the data within the page
>> -------------------------------------------------------------
>> Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
>> write to a page's data, unpin" can cause a problem. Case 5 may be considered a
>> superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
>> other words, if the code is neither Case 1 nor Case 2, it may still require
>> FOLL_PIN, for patterns like this:
>>
>> Correct (uses FOLL_PIN calls):
>>      pin_user_pages()
>>      write to the data within the pages
>>      unpin_user_pages()
> 
> Case 5 is crap though.  That bug should have been fixed by getting
> the locking right.  ie:
> 
> 	get_user_pages_fast();
> 	lock_page();
> 	kmap();
> 	set_bit();
> 	kunmap();
> 	set_page_dirty()
> 	unlock_page();
> 
> I should have vetoed that patch at the time, but I was busy with other things.
> 

It does seem like lock_page() is better, for now at least, because it
forces the kind of synchronization with file system writeback that is
still yet to be implemented for pin_user_pages().

Long term though, Case 5 provides an alternative way to do this
pattern--without using lock_page(). Also, note that Case 5, *in
general*, need not be done page-at-a-time, unlike the lock_page()
approach. Therefore, Case 5 might potentially help at some call sites,
either for deadlock avoidance or performance improvements.

In other words, once the other half of the pin_user_pages() plan is
implemented, either of these approaches should work.

Or, are you thinking that there is never a situation in which Case 5 is
valid?


thanks,
-- 
John Hubbard
NVIDIA