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Date:   Mon, 19 Nov 2018 11:27:52 -0700
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Jerome Glisse <jglisse@redhat.com>
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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