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[216.228.121.65]) by mx.google.com with ESMTPS id p1si400917pls.387.2019.03.28.16.34.05 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 28 Mar 2019 16:34:05 -0700 (PDT) Received-SPF: pass (google.com: domain of jhubbard@nvidia.com designates 216.228.121.65 as permitted sender) client-ip=216.228.121.65; Authentication-Results: mx.google.com; dkim=pass header.i=@nvidia.com header.s=n1 header.b=KIEVQmdN; spf=pass (google.com: domain of jhubbard@nvidia.com designates 216.228.121.65 as permitted sender) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=nvidia.com Received: from hqpgpgate101.nvidia.com (Not Verified[216.228.121.13]) by hqemgate16.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA) id ; Thu, 28 Mar 2019 16:34:03 -0700 Received: from hqmail.nvidia.com ([172.20.161.6]) by hqpgpgate101.nvidia.com (PGP Universal service); Thu, 28 Mar 2019 16:34:04 -0700 X-PGP-Universal: processed; by hqpgpgate101.nvidia.com on Thu, 28 Mar 2019 16:34:04 -0700 Received: from [10.110.48.28] (10.124.1.5) by HQMAIL101.nvidia.com (172.20.187.10) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Thu, 28 Mar 2019 23:34:04 +0000 Subject: Re: [PATCH v2 10/11] mm/hmm: add helpers for driver to safely take the mmap_sem v2 To: Jerome Glisse CC: , , Andrew Morton , Dan Williams References: <20190325144011.10560-11-jglisse@redhat.com> <9df742eb-61ca-3629-a5f4-8ad1244ff840@nvidia.com> <20190328213047.GB13560@redhat.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> <9e414b8c-0f98-a2f7-4f46-d335c015fc1b@nvidia.com> <20190328232404.GK13560@redhat.com> From: John Hubbard X-Nvconfidentiality: public Message-ID: Date: Thu, 28 Mar 2019 16:34:04 -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: <20190328232404.GK13560@redhat.com> X-Originating-IP: [10.124.1.5] X-ClientProxiedBy: HQMAIL104.nvidia.com (172.18.146.11) 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=1553816043; bh=a4JjKgKIVimDZFY8qHL1SLhhjDxQ2NbPDC8P33BMZJk=; 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=KIEVQmdNnxSUaWWXtj4v6m//bq05m4Ae6TWF56k0Vg7ToGeYQyMfo0VWX77dUdH/B 99UJHRAfwWYzFT3kq+zomaSW9OBE4NJj0RxjVSESnJQRhuIapuCoeGyCQ8E8PBZTWp xI8FWElX5ldyUScx1qHsAEy65QHcaOIkfyw8HwS35CEhwepVhr66Z3bj3eAYSN89Zw V/XEHCZfiHteuHYZ9Jd3cYiJRbayCQVeXrFAhLDvRQoEQB5yVTJuK8pKL7vGuTVpGN BLJu2uhNwhRHrVmusTKq9mtPftMxripQ0vJB6hA+A2KAMcaU2a9xwPCu+FeATEYLd2 jP36nbYD4ljDg== 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: On 3/28/19 4:24 PM, Jerome Glisse wrote: > On Thu, Mar 28, 2019 at 04:20:37PM -0700, John Hubbard wrote: >> 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 >>>>>> [...] >>>>>> OK, so let's either drop this patch, or if merge windows won't allow= that, >>>>>> then *eventually* drop this patch. And instead, put in a hmm_sanity_= check() >>>>>> that does the same checks. >>>>> >>>>> RDMA depends on this, so does the nouveau patchset that convert to ne= w API. >>>>> So i do not see reason to drop this. They are user for this they are = posted >>>>> and i hope i explained properly the benefit. >>>>> >>>>> It is a common pattern. Yes it only save couple lines of code but dow= n 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 lik= e they >>>> make things safer, but they don't actually do so. >>> >>> It is not about safety, sorry if it confused you but there is nothing a= bout >>> safety here, i can add a big fat comment that explains that there is no= safety >>> here. The intention is to allow the page fault handler that potential h= ave >>> hundred of page fault queue up to abort as soon as it sees that it is p= ointless >>> to keep faulting on a dying process. >>> >>> Again if we race it is _fine_ nothing bad will happen, we are just doin= g use- >>> less work that gonna be thrown on the floor and we are just slowing dow= n the >>> process tear down. >>> >> >> In addition to a comment, how about naming this thing to indicate the ab= ove=20 >> intention? I have a really hard time with this odd down_read() wrapper,= which >> allows code to proceed without really getting a lock. It's just too wron= g-looking. >> If it were instead named: >> >> hmm_is_exiting() >=20 > What about: hmm_lock_mmap_if_alive() ? >=20 That's definitely better, but I want to vote for just doing a check, not=20 taking any locks. I'm not super concerned about the exact name, but I really want a routine t= hat just checks (and optionally asserts, via WARN or BUG), and that's *all*. Th= en drivers can scatter that around like pixie dust as they see fit. Maybe righ= t before taking a lock, maybe in other places. Decoupled from locking. thanks, --=20 John Hubbard NVIDIA