Received: by 2002:a25:5b86:0:0:0:0:0 with SMTP id p128csp971633ybb;
        Thu, 28 Mar 2019 16:21:55 -0700 (PDT)
X-Google-Smtp-Source: APXvYqxV5vWx6bMZYQa2QA9QYQvwvCHysP7FBaCgknQeDuTE+sK4WGtpwrDBnBQqdStB0/twUTQm
X-Received: by 2002:a17:902:1a9:: with SMTP id b38mr45461636plb.37.1553815315043;
        Thu, 28 Mar 2019 16:21:55 -0700 (PDT)
ARC-Seal: i=1; a=rsa-sha256; t=1553815315; cv=none;
        d=google.com; s=arc-20160816;
        b=zNB+SrAE6qLDeaa4wnBL58UetwybPLGkiGu7BqJJkFkE4FJPI+0QH+BQrsPJ5xcro8
         vgMFTB2pAPv6j8XS2fjiR/Hb6j4mmHMUG77FzQL/v61lcV1BXzYvwa1U3tubeetH8hlM
         hNMfC0twZ4s6fPGxTT9X8TuPSn5CScX9dV4gPiLdyr9m3S97zNC7ZgQ7XQaqNWlhXQol
         7cGAt2M37CSGMLq6gbOLZz2HosWkO3A4AnQdCJE5xmujRkliH9GgMjbtjnQRX1bUFzFK
         SA5aW8YnnPF8PvJMPSiTIQnoNhG9vKZ1Ftp2ZWG3dInD8KQK2rmNR2uQ00SUC+2vWPrr
         Zp9g==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816;
        h=list-id:precedence:sender:dkim-signature:content-transfer-encoding
         :content-language:in-reply-to:mime-version:user-agent:date
         :message-id:from:references:cc:to:subject;
        bh=cwvfXAmYS2xpbe/FjvzNojKYMDG/S4Ny/qLUa3K9/HA=;
        b=1GISYd1fsgy48yoEBXgqAZqi4U2Nvq4v2fDELFCjEri7+0Gg85qa6yFw3VwTjVCbav
         4lM3GzI60ZA0Zz3OBeomUHHAkSA6eyJnaVlD/TUIUWAjACLAU3O6wt7Ip/TvzONCWXQF
         eGLmz5FJiBLs+a4NfHAfwavcQz/veKHE2BbrlobyudcLmiFB8kdhHT/yuS9sQ7kxcuW6
         V2rZq60AkUQPhB7N1qnweT/Nur86rL4gcX3/NKxSXfd9ukUynJHGBDcamau5qoncek3n
         q9fvAEroZDqCo3livZ/RxAM49c+3V6dMT7pgGefVpcibYUguAZofsQiJ3MH8GW/OY90S
         UVsg==
ARC-Authentication-Results: i=1; mx.google.com;
       dkim=pass header.i=@nvidia.com header.s=n1 header.b=kNMJMs9C;
       spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org;
       dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=nvidia.com
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67])
        by mx.google.com with ESMTP id h7si304110pfb.167.2019.03.28.16.21.39;
        Thu, 28 Mar 2019 16:21:55 -0700 (PDT)
Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Authentication-Results: mx.google.com;
       dkim=pass header.i=@nvidia.com header.s=n1 header.b=kNMJMs9C;
       spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org;
       dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=nvidia.com
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1728544AbfC1XUj (ORCPT <rfc822;carmate383@gmail.com>
        + 99 others); Thu, 28 Mar 2019 19:20:39 -0400
Received: from hqemgate14.nvidia.com ([216.228.121.143]:6479 "EHLO
        hqemgate14.nvidia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1727658AbfC1XUj (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 28 Mar 2019 19:20:39 -0400
Received: from hqpgpgate101.nvidia.com (Not Verified[216.228.121.13]) by hqemgate14.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA)
        id <B5c9d56c90000>; Thu, 28 Mar 2019 16:20:41 -0700
Received: from hqmail.nvidia.com ([172.20.161.6])
  by hqpgpgate101.nvidia.com (PGP Universal service);
  Thu, 28 Mar 2019 16:20:38 -0700
X-PGP-Universal: processed;
        by hqpgpgate101.nvidia.com on Thu, 28 Mar 2019 16:20:38 -0700
Received: from [10.110.48.28] (172.20.13.39) by HQMAIL101.nvidia.com
 (172.20.187.10) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Thu, 28 Mar
 2019 23:20:38 +0000
Subject: Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take
 the mmap_sem v2
To:     Jerome Glisse <jglisse@redhat.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>
References: <20190325144011.10560-1-jglisse@redhat.com>
 <20190325144011.10560-11-jglisse@redhat.com>
 <9df742eb-61ca-3629-a5f4-8ad1244ff840@nvidia.com>
 <20190328213047.GB13560@redhat.com>
 <a16efd42-3e2b-1b72-c205-0c2659de2750@nvidia.com>
 <20190328220824.GE13560@redhat.com>
 <068db0a8-fade-8ed1-3b9d-c29c27797301@nvidia.com>
 <20190328224032.GH13560@redhat.com>
 <0b698b36-da17-434b-b8e7-4a91ac6c9d82@nvidia.com>
 <20190328230543.GI13560@redhat.com>
From:   John Hubbard <jhubbard@nvidia.com>
X-Nvconfidentiality: public
Message-ID: <9e414b8c-0f98-a2f7-4f46-d335c015fc1b@nvidia.com>
Date:   Thu, 28 Mar 2019 16:20:37 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.5.3
MIME-Version: 1.0
In-Reply-To: <20190328230543.GI13560@redhat.com>
X-Originating-IP: [172.20.13.39]
X-ClientProxiedBy: HQMAIL106.nvidia.com (172.18.146.12) To
 HQMAIL101.nvidia.com (172.20.187.10)
Content-Type: text/plain; charset="utf-8"
Content-Language: en-US-large
Content-Transfer-Encoding: quoted-printable
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1;
        t=1553815241; bh=cwvfXAmYS2xpbe/FjvzNojKYMDG/S4Ny/qLUa3K9/HA=;
        h=X-PGP-Universal:Subject:To:CC:References:From:X-Nvconfidentiality:
         Message-ID:Date:User-Agent:MIME-Version:In-Reply-To:
         X-Originating-IP:X-ClientProxiedBy:Content-Type:Content-Language:
         Content-Transfer-Encoding;
        b=kNMJMs9C+TM1nNFTqcCV7IrVAkqxMYU++6lQG9ATbs5dJW3bohUET58V2LGYBJ3AS
         BJOR3Wxcqu+O47jZVuo4jxeNgexPD12CKcgQqGkcjnMg7a14DNYwvxMfNWRsSBQLEg
         PmVuCr8J6jmZ9N9JV6HxPfnhkC4PZ0lRQyjpXcUJ94q9EVm/ibBxc6wIgOfAefZI5U
         ALdxLZSvIEDDguQNaUCnjr01WMaGnRhvhwa+gudD1FXzyCnmBQ83NvG4D7CG5Blw03
         fRfwFvygK2kgIdrvJZK3kRGV9HlIEiUvIRIqJV8go1+5AoYHs0j3fwtpwlXsSukM1Z
         wd9dFJcLPiWiA==
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 3/28/19 4:05 PM, Jerome Glisse wrote:
> On Thu, Mar 28, 2019 at 03:43:33PM -0700, John Hubbard wrote:
>> On 3/28/19 3:40 PM, Jerome Glisse wrote:
>>> On Thu, Mar 28, 2019 at 03:25:39PM -0700, John Hubbard wrote:
>>>> On 3/28/19 3:08 PM, Jerome Glisse wrote:
>>>>> On Thu, Mar 28, 2019 at 02:41:02PM -0700, John Hubbard wrote:
>>>>>> On 3/28/19 2:30 PM, Jerome Glisse wrote:
>>>>>>> On Thu, Mar 28, 2019 at 01:54:01PM -0700, John Hubbard wrote:
>>>>>>>> On 3/25/19 7:40 AM, jglisse@redhat.com wrote:
>>>>>>>>> From: J=C3=A9r=C3=B4me Glisse <jglisse@redhat.com>
>>>> [...]
>>>> OK, so let's either drop this patch, or if merge windows won't allow t=
hat,
>>>> then *eventually* drop this patch. And instead, put in a hmm_sanity_ch=
eck()
>>>> that does the same checks.
>>>
>>> RDMA depends on this, so does the nouveau patchset that convert to new =
API.
>>> So i do not see reason to drop this. They are user for this they are po=
sted
>>> and i hope i explained properly the benefit.
>>>
>>> It is a common pattern. Yes it only save couple lines of code but down =
the
>>> road i will also help for people working on the mmap_sem patchset.
>>>
>>
>> It *adds* a couple of lines that are misleading, because they look like =
they
>> make things safer, but they don't actually do so.
>=20
> It is not about safety, sorry if it confused you but there is nothing abo=
ut
> safety here, i can add a big fat comment that explains that there is no s=
afety
> here. The intention is to allow the page fault handler that potential hav=
e
> hundred of page fault queue up to abort as soon as it sees that it is poi=
ntless
> to keep faulting on a dying process.
>=20
> Again if we race it is _fine_ nothing bad will happen, we are just doing =
use-
> less work that gonna be thrown on the floor and we are just slowing down =
the
> process tear down.
>=20

In addition to a comment, how about naming this thing to indicate the above=
=20
intention?  I have a really hard time with this odd down_read() wrapper, wh=
ich
allows code to proceed without really getting a lock. It's just too wrong-l=
ooking.
If it were instead named:

	hmm_is_exiting()

and had a comment about why racy is OK, then I'd be a lot happier. :)


thanks,
--=20
John Hubbard
NVIDIA