From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=jC02=BN=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=-7.1 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 D88AAC433DF
	for <linux-mm@archiver.kernel.org>; Mon,  3 Aug 2020 03:28:12 +0000 (UTC)
Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17])
	by mail.kernel.org (Postfix) with ESMTP id 7609A206F6
	for <linux-mm@archiver.kernel.org>; Mon,  3 Aug 2020 03:28:12 +0000 (UTC)
Authentication-Results: mail.kernel.org;
	dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="T3YaTWWL"
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7609A206F6
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 3F6E38D00D3; Sun,  2 Aug 2020 23:28:12 -0400 (EDT)
Received: by kanga.kvack.org (Postfix, from userid 40)
	id 3A6A48D00AA; Sun,  2 Aug 2020 23:28:12 -0400 (EDT)
X-Delivered-To: int-list-linux-mm@kvack.org
Received: by kanga.kvack.org (Postfix, from userid 63042)
	id 2953B8D00D3; Sun,  2 Aug 2020 23:28:12 -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 131A48D00AA
	for <linux-mm@kvack.org>; Sun,  2 Aug 2020 23:28:12 -0400 (EDT)
Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251])
	by forelay02.hostedemail.com (Postfix) with ESMTP id 88BD43620
	for <linux-mm@kvack.org>; Mon,  3 Aug 2020 03:28:11 +0000 (UTC)
X-FDA: 77107823982.15.coat84_420e9a426f9a
Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251])
	by smtpin15.hostedemail.com (Postfix) with ESMTP id 5772D1814B0C8
	for <linux-mm@kvack.org>; Mon,  3 Aug 2020 03:28:11 +0000 (UTC)
X-HE-Tag: coat84_420e9a426f9a
X-Filterd-Recvd-Size: 5119
Received: from hqnvemgate26.nvidia.com (hqnvemgate26.nvidia.com [216.228.121.65])
	by imf31.hostedemail.com (Postfix) with ESMTP
	for <linux-mm@kvack.org>; Mon,  3 Aug 2020 03:28:10 +0000 (UTC)
Received: from hqpgpgate101.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate26.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA)
	id <B5f27843b0000>; Sun, 02 Aug 2020 20:27:55 -0700
Received: from hqmail.nvidia.com ([172.20.161.6])
  by hqpgpgate101.nvidia.com (PGP Universal service);
  Sun, 02 Aug 2020 20:28:09 -0700
X-PGP-Universal: processed;
	by hqpgpgate101.nvidia.com on Sun, 02 Aug 2020 20:28:09 -0700
Received: from [10.2.58.124] (10.124.1.5) by HQMAIL107.nvidia.com
 (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Mon, 3 Aug
 2020 03:28:08 +0000
Subject: Re: [PATCH] mm: introduce reference pages
To: Peter Collingbourne <pcc@google.com>, Andrew Morton
	<akpm@linux-foundation.org>, Catalin Marinas <catalin.marinas@arm.com>
CC: Evgenii Stepanov <eugenis@google.com>,
	<linux-arm-kernel@lists.infradead.org>, <linux-mm@kvack.org>
References: <20200731203241.50427-1-pcc@google.com>
From: John Hubbard <jhubbard@nvidia.com>
Message-ID: <92fa4a71-d8dc-0f07-832c-cbceca43e537@nvidia.com>
Date: Sun, 2 Aug 2020 20:28:08 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <20200731203241.50427-1-pcc@google.com>
X-Originating-IP: [10.124.1.5]
X-ClientProxiedBy: HQMAIL111.nvidia.com (172.20.187.18) To
 HQMAIL107.nvidia.com (172.20.187.13)
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1;
	t=1596425275; bh=to3PWnCaeBvNqUxrIlIx1/WQ3rw7XROT9vzswJSETpU=;
	h=X-PGP-Universal:Subject:To:CC:References:From:Message-ID:Date:
	 User-Agent:MIME-Version:In-Reply-To:X-Originating-IP:
	 X-ClientProxiedBy:Content-Type:Content-Language:
	 Content-Transfer-Encoding;
	b=T3YaTWWLp1kixcjQgNOXuUZbtvEwm9z/lZE5GMolMF9tlKN8ut0SQe+6QIGAtbnwG
	 xr1RxDwiv6ToFL4Fw1sxXcuL5PqbYC97Pk2j2/GM7fb9eK8RI7wCeClmODnB7Z+ny4
	 3m38u0Fadd1erRB+BQGve+s+yw5i3STBvegY/xfwaxTpdxuEDKuTwza1pNYZlaGkao
	 X0XsHlC0oDHIo78gE2xR9SmwTln0fBNaJvnB7Z1dyx1ygJX3OTZJ/Z5hH/B0kfnFjj
	 W83uksDqqCxTijJaCJwPgPMI61Y0i3iO0BUjJPOoS7cxu6ujl3hkUQBZJiAeE+IHyN
	 ftkIB9rtJ/G5g==
X-Rspamd-Queue-Id: 5772D1814B0C8
X-Spamd-Result: default: False [0.00 / 100.00]
X-Rspamd-Server: rspam01
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 7/31/20 1:32 PM, Peter Collingbourne wrote:
...

Hi,

I can see why you want to do this. A few points to consider, below.

btw, the patch would *not* apply for me, via `git am`. I finally used
patch(1) and that worked. Probably good to mention which tree and branch
this applies to, as a first step to avoiding that, but I'm not quite sure
what else went wrong. Maybe use stock git, instead of
2.28.0.163.g6104cc2f0b6-goog? Just guessing.

> @@ -1684,9 +1695,33 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
>   	return (vm_flags & (VM_NORESERVE | VM_SHARED | VM_WRITE)) == VM_WRITE;
>   }
>   
> +static vm_fault_t refpage_fault(struct vm_fault *vmf)
> +{
> +	struct page *page;
> +
> +	if (get_user_pages((unsigned long)vmf->vma->vm_private_data, 1, 0,
> +			   &page, 0) != 1)
> +		return VM_FAULT_SIGSEGV;
> +

This will end up overflowing the page->_refcount in some situations.

Some thoughts:

In order to implement this feature, the reference pages need to be made
at least a little bit more special, and probably little bit more like
zero pages. At one extreme, for example, zero pages could be a special
case of reference pages, although I'm not sure of a clean way to
implement that.


The reason that more special-ness is required, is that things such as
reference counting and locking can be special-cased with zero pages.
Doing so allows avoiding page->_refcount overflows, for example. Your
patch here, however, allows normal pages to be treated *almost* like a
zero page, in that it's a page full of constant value data. But because
a refpage can be any page, not just a special one that is defined at a
single location, that leads to problems with refcounts.


> +	vmf->page = page;
> +	return VM_FAULT_LOCKED;

Is the page really locked, or is this a case of "the page is special and
we can safely claim it is locked"? Maybe I'm just confused about the use
of VM_FAULT_LOCKED: I thought you only should set it after locking the
page.


> +}
> +
> +static void refpage_close(struct vm_area_struct *vma)
> +{
> +	/* This function exists only to prevent is_mergeable_vma from allowing a
> +	 * reference page mapping to be merged with an anonymous mapping.
> +	 */

While it is true that implementing a vma's .close() method will prevent
vma merging, this is an abuse of that function: it depends on how that
function is implemented. And given that refpages represent significant
new capability, I think they deserve their own "if" clause (and perhaps
a VMA flag) in is_mergeable_vma(), instead of this kind of minor hack.



thanks,
-- 
John Hubbard
NVIDIA