linux-kernel - Re: [PATCH v1 04/12] mm/rmap: implement make_device_exclusive() using folio

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Message-ID: <9f59b475-232f-41d4-bd6f-7f84111062ac@redhat.com>
Date: Thu, 30 Jan 2025 16:59:16 +0100
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-mm@...ck.org, nouveau@...ts.freedesktop.org,
 Andrew Morton <akpm@...ux-foundation.org>, Jérôme Glisse
 <jglisse@...hat.com>, Jonathan Corbet <corbet@....net>,
 Alex Shi <alexs@...nel.org>, Yanteng Si <si.yanteng@...ux.dev>,
 Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pasha Tatashin <pasha.tatashin@...een.com>, Peter Xu <peterx@...hat.com>,
 Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1 04/12] mm/rmap: implement make_device_exclusive() using
 folio_walk instead of rmap walk

>>>>> Note that the PTE is
>>>>> always writable, and we can always create a writable-device-exclusive
>>>>> entry.
>>>>>
>>>>> With this change, device-exclusive is fully compatible with THPs /
>>>>> large folios. We still require PMD-sized THPs to get PTE-mapped, and
>>>>> supporting PMD-mapped THP (without the PTE-remapping) is a different
>>>>> endeavour that might not be worth it at this point.
>>>
>>> I'm not sure we actually want hugepages for device exclusive, since it has
>>> an impact on what's allowed and what not. If we only ever do 4k entries
>>> then userspace can assume that as long atomics are separated by a 4k page
>>> there's no issue when both the gpu and cpu hammer on them. If we try to
>>> keep thp entries then suddenly a workload that worked before will result
>>> in endless ping-pong between gpu and cpu because the separate atomic
>>> counters (or whatever) now all sit in the same 2m page.
>>
>> Agreed. And the conversion + mapping into the device gets trickier.
>>
>>>
>>> So going with thp might result in userspace having to spread out atomics
>>> even more, which is just wasting memory and not saving any tlb entries
>>> since often you don't need that many.
>>>
>>> tldr; I think not supporting thp entries for device exclusive is a
>>> feature, not a bug.
>>
>> So, you agree with my "different endeavour that might not be worth it"
>> statement?
> 
> Yes.
> 
> Well I think we should go further and clearly document that we
> intentionally return split pages. Because it's part of the uapi contract
> with users of all this.

Yes, see my reply to patch #3/

> 
> And if someone needs pmd entries for performance or whatever, we need two
> things:
> 
> a) userspace must mmap that memory as hugepage memory, to clearly signal
> the promise that atomics are split up on hugepage sizes and not just page
> size
> 
> b) we need to extend make_device_exclusive and drivers to handle the
> hugetlb folio case
> 
> I think thp is simply not going to work here, it's impossible (without
> potentially causing fault storms) to figure out what userspace might want.

Right, I added a link to this discussion in the patch.

Thanks!

-- 
Cheers,

David / dhildenb