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Subject: Re: [RESEND PATCH 2/3] nouveau: fix mixed normal and device private
 page migration
To:     Jason Gunthorpe <jgg@ziepe.ca>
CC:     Christoph Hellwig <hch@lst.de>, <nouveau@lists.freedesktop.org>,
        <linux-kernel@vger.kernel.org>, Jerome Glisse <jglisse@redhat.com>,
        "John Hubbard" <jhubbard@nvidia.com>,
        Ben Skeggs <bskeggs@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, Bharata B Rao <bharata@linux.ibm.com>
References: <20200622233854.10889-1-rcampbell@nvidia.com>
 <20200622233854.10889-3-rcampbell@nvidia.com> <20200624072355.GB18609@lst.de>
 <330f6a82-d01d-db97-1dec-69346f41e707@nvidia.com>
 <a9aba057-3786-8204-f782-6e8f3c290b35@nvidia.com>
 <20200625173144.GT6578@ziepe.ca>
From:   Ralph Campbell <rcampbell@nvidia.com>
Message-ID: <412b3a51-619a-4f94-da96-f4596e5eb510@nvidia.com>
Date:   Thu, 25 Jun 2020 10:42:24 -0700
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On 6/25/20 10:31 AM, Jason Gunthorpe wrote:
> On Thu, Jun 25, 2020 at 10:25:38AM -0700, Ralph Campbell wrote:
>> Making sure to include linux-mm and Bharata B Rao for IBM's
>> use of migrate_vma*().
>>
>> On 6/24/20 11:10 AM, Ralph Campbell wrote:
>>>
>>> On 6/24/20 12:23 AM, Christoph Hellwig wrote:
>>>> On Mon, Jun 22, 2020 at 04:38:53PM -0700, Ralph Campbell wrote:
>>>>> The OpenCL function clEnqueueSVMMigrateMem(), without any flags, will
>>>>> migrate memory in the given address range to device private memory. T=
he
>>>>> source pages might already have been migrated to device private memor=
y.
>>>>> In that case, the source struct page is not checked to see if it is
>>>>> a device private page and incorrectly computes the GPU's physical
>>>>> address of local memory leading to data corruption.
>>>>> Fix this by checking the source struct page and computing the correct
>>>>> physical address.
>>>>
>>>> I'm really worried about all this delicate code to fix the mixed
>>>> ranges.=C2=A0 Can't we make it clear at the migrate_vma_* level if we =
want
>>>> to migrate from or two device private memory, and then skip all the wo=
rk
>>>> for regions of memory that already are in the right place?=C2=A0 This =
might be
>>>> a little more work initially, but I think it leads to a much better
>>>> API.
>>>>
>>>
>>> The current code does encode the direction with src_owner !=3D NULL mea=
ning
>>> device private to system memory and src_owner =3D=3D NULL meaning syste=
m
>>> memory to device private memory. This patch would obviously defeat that
>>> so perhaps a flag could be added to the struct migrate_vma to indicate =
the
>>> direction but I'm unclear how that makes things less delicate.
>>> Can you expand on what you are worried about?
>>>
>>> The issue with invalidations might be better addressed by letting the d=
evice
>>> driver handle device private page TLB invalidations when migrating to
>>> system memory and changing migrate_vma_setup() to only invalidate CPU
>>> TLB entries for normal pages being migrated to device private memory.
>>> If a page isn't migrating, it seems inefficient to invalidate those TLB
>>> entries.
>>>
>>> Any other suggestions?
>>
>> After a night's sleep, I think this might work. What do others think?
>>
>> 1) Add a new MMU_NOTIFY_MIGRATE enum to mmu_notifier_event.
>>
>> 2) Change migrate_vma_collect() to use the new MMU_NOTIFY_MIGRATE event =
type.
>>
>> 3) Modify nouveau_svmm_invalidate_range_start() to simply return (no inv=
alidations)
>> for MMU_NOTIFY_MIGRATE mmu notifier callbacks.
>=20
> Isn't it a bit of an assumption that migrate_vma_collect() is only
> used by nouveau itself?
>=20
> What if some other devices' device_private pages are being migrated?
>=20
> Jason
>=20

Good point. The driver needs a way of knowing the callback is due its call
to migrate_vma_setup() and not some other migration invalidation.
How about adding a void pointer to struct mmu_notifier_range
which migrate_vma_collect() can set to src_owner. If the event is
MMU_NOTIFY_MIGRATE and the src_owner matches the void pointer, then the
callback should be the one the driver initiated.