Re: [Question] Are "device exclusive non-swap entries" / "SVM atomics in Nouveau" still getting used in practice?

[David Hildenbrand <david@redhat.com>]

On 23.01.25 16:08, Simona Vetter wrote:
> On Thu, Jan 23, 2025 at 11:20:37AM +0100, David Hildenbrand wrote:
>> Hi,
>>
>> I keep finding issues in our implementation of "device exclusive non-swap
>> entries", and the way it messes with mapcounts is disgusting.
>>
>> As a reminder, what we do here is to replace a PTE pointing to an anonymous
>> page by a "device exclusive non-swap entry".
>>
>> As long as the original PTE is in place, the only CPU can access it, as soon
>> as the "device exclusive non-swap entry" is in place, only the device can
>> access it. Conversion back and forth is triggered by CPU / device faults.
>>
>> I have fixes/reworks/simplifications for most things, but as there is only a
>> "real" single user in-tree of make_device_exclusive():
>>
>> 	drivers/gpu/drm/nouveau/nouveau_svm.c
>>
>> to "support SVM atomics in Nouveau [1]"
>>
>> naturally I am wondering: is this still a thing on actual hardware, or is it
>> already stale on recent hardware and not really required anymore?
>>
>>
>> [1] https://lore.kernel.org/linux-kernel//6621654.gmDyfcmpjF@nvdebian/T/
> 

Thanks for your answer!

Nvidia folks told me on a different channel that it's still getting used.

> As long as you don't have a coherent interconnect it's needed. On intel
> discrete device atomics require device memory, so they need full hmm
> migration (and hence wont use this function even once we land intel gpu
> svm code in upstream).

Makes sense.

> On integrated the gpu is tied into the coherency
> fabric, so there it's not needed.
> 
> I think the more fundamental question with both this function here and
> with forced migration to device memory is that there's no guarantee it
> will work out.

Yes, in particular with device-exclusive, it doesn't really work with 
THP and is only limited to anonymous memory. I have patches to at least 
make it work reliably with THP.

Then, we seem to give up too easily if we cannot lock the folio when 
wanting to convert to device-exclusive, which also looks rather odd. But 
well, maybe it just works good enough in the common case, or there is 
some other retry logic that makes it fly.

> At least that's my understanding. And for this gpu device
> atomics without coherent interconnect idea to work, we'd need to be able
> to guarantee that we can make any page device exclusive. So from my side I
> have some pretty big question marks on this entire thing overall.

I don't think other memory (shmem/file/...) is really feasible as soon 
as other processes (not the current process) map/write/read file pages.

We could really only handle if we converted a single PTE and that PTE is 
getting converted back again.

There are other concerns I have (what if the page is pinned and access 
outside of the user space page tables?). Maybe there was not need to 
handle these cases so far.

So best I can do is make anonymous memory more reliable with 
device-exclusive and fixup some of the problematic parts that I see 
(e.g., broken page reclaim, page migration, ...).

But before starting to cleanup+improve the existing handling of 
anonymous memory, I was wondering if this whole thing is getting used at 
all.

-- 
Cheers,

David / dhildenb