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[174.3.196.123]) by smtp.gmail.com with ESMTPSA id b81-v6sm50285206pfj.183.2018.11.19.10.27.53 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 19 Nov 2018 10:27:53 -0800 (PST) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1gOoH2-0004qA-Nt; Mon, 19 Nov 2018 11:27:52 -0700 Date: Mon, 19 Nov 2018 11:27:52 -0700 From: Jason Gunthorpe To: Jerome Glisse Cc: Leon Romanovsky , Kenneth Lee , Tim Sell , linux-doc@vger.kernel.org, Alexander Shishkin , Zaibo Xu , zhangfei.gao@foxmail.com, linuxarm@huawei.com, haojian.zhuang@linaro.org, Christoph Lameter , Hao Fang , Gavin Schenk , RDMA mailing list , Zhou Wang , Doug Ledford , Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= , David Kershner , Kenneth Lee , Johan Hovold , Cyrille Pitchen , Sagar Dharia , Jens Axboe , guodong.xu@linaro.org, linux-netdev , Randy Dunlap , linux-kernel@vger.kernel.org, Vinod Koul , linux-crypto@vger.kernel.org, Philippe Ombredanne , Sanyog Kale , "David S. Miller" , linux-accelerators@lists.ozlabs.org Subject: Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce Message-ID: <20181119182752.GA4890@ziepe.ca> References: <20181112075807.9291-2-nek.in.cn@gmail.com> <20181113002354.GO3695@mtr-leonro.mtl.com> <95310df4-b32c-42f0-c750-3ad5eb89b3dd@gmail.com> <20181114160017.GI3759@mtr-leonro.mtl.com> <20181115085109.GD157308@Turing-Arch-b> <20181115145455.GN3759@mtr-leonro.mtl.com> <20181119091405.GE157308@Turing-Arch-b> <20181119091910.GF157308@Turing-Arch-b> <20181119104801.GF8268@mtr-leonro.mtl.com> <20181119164853.GA4593@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181119164853.GA4593@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 19, 2018 at 11:48:54AM -0500, Jerome Glisse wrote: > Just to comment on this, any infiniband driver which use umem and do > not have ODP (here ODP for me means listening to mmu notifier so all > infiniband driver except mlx5) will be affected by same issue AFAICT. > > AFAICT there is no special thing happening after fork() inside any of > those driver. So if parent create a umem mr before fork() and program > hardware with it then after fork() the parent might start using new > page for the umem range while the old memory is use by the child. The > reverse is also true (parent using old memory and child new memory) > bottom line you can not predict which memory the child or the parent > will use for the range after fork(). > > So no matter what you consider the child or the parent, what the hw > will use for the mr is unlikely to match what the CPU use for the > same virtual address. In other word: > > Before fork: > CPU parent: virtual addr ptr1 -> physical address = 0xCAFE > HARDWARE: virtual addr ptr1 -> physical address = 0xCAFE > > Case 1: > CPU parent: virtual addr ptr1 -> physical address = 0xCAFE > CPU child: virtual addr ptr1 -> physical address = 0xDEAD > HARDWARE: virtual addr ptr1 -> physical address = 0xCAFE > > Case 2: > CPU parent: virtual addr ptr1 -> physical address = 0xBEEF > CPU child: virtual addr ptr1 -> physical address = 0xCAFE > HARDWARE: virtual addr ptr1 -> physical address = 0xCAFE IIRC this is solved in IB by automatically calling madvise(MADV_DONTFORK) before creating the MR. MADV_DONTFORK .. This is useful to prevent copy-on-write semantics from changing the physical location of a page if the parent writes to it after a fork(2) .. Jason