linux-kernel - Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers

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Date:   Wed, 19 Feb 2020 17:10:45 -0800
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
CC:     "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        "John Hubbard" <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Skeggs <bskeggs@...hat.com>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers

On 1/16/20 12:21 PM, Jason Gunthorpe wrote:
> On Thu, Jan 16, 2020 at 12:16:30PM -0800, Ralph Campbell wrote:
>> Can you point me to the latest ODP code? Seems like my understanding is
>> quite off.
> 
> https://elixir.bootlin.com/linux/v5.5-rc6/source/drivers/infiniband/hw/mlx5/odp.c
> 
> Look for the word 'implicit'
> 
> mlx5_ib_invalidate_range() releases the interval_notifier when there are
> no populated shadow PTEs in its leaf
> 
> pagefault_implicit_mr() creates an interval_notifier that covers the
> level in the page table that needs population. Notice it just uses an
> unlocked xa_load to find the page table level.
> 
> The locking is pretty tricky as it relies on RCU, but the fault flow
> is fairly lightweight.
> 
> Jason
> 
Thanks for the information, Jason.

I'm still interested in finding a way to support range based hints to device drivers.
madvise() looks like it only sets a bit in vma->vm_flags or acts on the
advice immediately. mbind() and set_mempolicy() only work with CPUs and memory
with NUMA a node number. What I'm looking for is a way for the device to know
whether to migrate pages to device private memory on a fault, whether to duplicate
read-only pages in device private memory, or remote map/access a page instead of migrating it.
For example, there is a working draft extension to OpenCL,
https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/USM/cl_intel_unified_shared_memory.asciidoc
that could provide a way to specify this sort of advice.
C++ is also looking at extentions for specifying affinity attributes.
In any case, these are probably a long ways off before being finalized and implemented.

I also have some changes to support THP migration to device private memory but that
will require updating nouveau to use 2MB TLB mappings.

In the mean time, I can update the HMM self tests to do something like ODP without
changing mm/mmu_notifier.c but I don't think I can easily change nouveau to that model.