linux-kernel - Re: [PATCH v1 08/12] mm/rmap: handle device-exclusive entries correctly in try_to_unmap

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Message-ID: <0b3a38cc-b956-4db4-8b13-c8180926d2a3@redhat.com>
Date: Thu, 30 Jan 2025 16:52:27 +0100
From: David Hildenbrand <david@...hat.com>
To: 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>,
 Alistair Popple <apopple@...dia.com>, Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1 08/12] mm/rmap: handle device-exclusive entries
 correctly in try_to_unmap_one()

On 30.01.25 14:06, Simona Vetter wrote:
> On Thu, Jan 30, 2025 at 12:08:42PM +0100, David Hildenbrand wrote:
>> On 30.01.25 11:10, Simona Vetter wrote:
>>> On Wed, Jan 29, 2025 at 12:54:06PM +0100, David Hildenbrand wrote:
>>>> Ever since commit b756a3b5e7ea ("mm: device exclusive memory access")
>>>> we can return with a device-exclusive entry from page_vma_mapped_walk().
>>>>
>>>> try_to_unmap_one() is not prepared for that, so teach it about these
>>>> non-present nonswap PTEs.
>>>>
>>>> Before that, could we also have triggered this case with device-private
>>>> entries? Unlikely.
>>>
>>> Just quick comment on this, I'm still pondering all the other aspects.
>>>
>>> device-private memory is entirely owned by the driver, the core mm isn't
>>> supposed to touch these beyond migrating it back to system memory in
>>> do_swap_page. Plus using rmap when the driver asks for invalidating
>>> mappings as needed.
>>>
>>> So no lru, thp, migration or anything initiated by core mm should ever
>>> happen on these device private pages. If it does, it'd be a bug.
>>
>> I was not 100% sure about HWPoison handling, that's why I added that
>> comment. In other regards I agree: reclaim etc. does not apply.
> 
> So maybe I'm just entirely lost, but unless you have a coherent
> interconnect I don't think hwpoisin should get involved with device
> private memory? And for a coherent interconnect it's just device memory,
> which isn't treated very special.

I would have thought that in a scenario Jason describes, that you would 
still want to zap the page from the page table (try_to_unmap()) and 
install a hwpoison entry instead.

But yes, right now this should never ever happen: memory_failure() does 
some ZONE_DEVICE specific things, but likely doesn't call try_to_unmap() 
on these folios.

> 
> Also to clarify, I meant this as a general comment for all subsequent
> patches that have the same paragraph.

Yeah, I'll rephrase that to "We'll never hit that case for special 
device-private pages."

-- 
Cheers,

David / dhildenb