Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce

Leon Romanovsky
2018-11-19T19:02:40Z

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On Mon, Nov 19, 2018 at 01:42:16PM -0500, Jerome Glisse wrote:
> On Mon, Nov 19, 2018 at 11:27:52AM -0700, Jason Gunthorpe wrote:
> > 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) ..
>
> This would work around the issue but this is not transparent ie
> range marked with DONTFORK no longer behave as expected from the
> application point of view.
>
> Also it relies on userspace doing the right thing (which is not
> something i usualy trust :)).

The good thing that we didn't see anyone who succeeded to run
IB stack without our user space, which does right thing under
the hood :).

>
> Cheers,
> Jérôme

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