Date: Mon, 19 Nov 2018 11:27:52 -0700
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Jerome Glisse <jglisse@redhat.com>
Cc: Leon Romanovsky <leon@kernel.org>,
        Kenneth Lee <liguozhu@hisilicon.com>,
        Tim Sell <timothy.sell@unisys.com>, linux-doc@vger.kernel.org,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Zaibo Xu <xuzaibo@huawei.com>, zhangfei.gao@foxmail.com,
        linuxarm@huawei.com, haojian.zhuang@linaro.org,
        Christoph Lameter <cl@linux.com>,
        Hao Fang <fanghao11@huawei.com>,
        Gavin Schenk <g.schenk@eckelmann.de>,
        RDMA mailing list <linux-rdma@vger.kernel.org>,
        Zhou Wang <wangzhou1@hisilicon.com>,
        Doug Ledford <dledford@redhat.com>,
        Uwe =?utf-8?Q?Kleine-K=C3=B6nig?=
        <u.kleine-koenig@pengutronix.de>,
        David Kershner <david.kershner@unisys.com>,
        Kenneth Lee <nek.in.cn@gmail.com>,
        Johan Hovold <johan@kernel.org>,
        Cyrille Pitchen <cyrille.pitchen@free-electrons.com>,
        Sagar Dharia <sdharia@codeaurora.org>,
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        linux-netdev <netdev@vger.kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
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        Philippe Ombredanne <pombredanne@nexb.com>,
        Sanyog Kale <sanyog.r.kale@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        linux-accelerators@lists.ozlabs.org
Subject: Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce
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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