linux-kernel - Re: [PATCH v1 05/12] mm/memory: detect writability in restore_exclusive_pte() through can_change_pte

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Message-ID: <2670f65f-e973-483e-aed6-526d00125ad7@redhat.com>
Date: Thu, 30 Jan 2025 10:58:51 +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 05/12] mm/memory: detect writability in
 restore_exclusive_pte() through can_change_pte_writable()

On 30.01.25 10:51, Simona Vetter wrote:
> On Wed, Jan 29, 2025 at 12:54:03PM +0100, David Hildenbrand wrote:
>> Let's do it just like mprotect write-upgrade or during NUMA-hinting
>> faults on PROT_NONE PTEs: detect if the PTE can be writable by using
>> can_change_pte_writable().
>>
>> Set the PTE only dirty if the folio is dirty: we might not
>> necessarily have a write access, and setting the PTE writable doesn't
>> require setting the PTE dirty.
> 
> Not sure whether there's much difference in practice, since a device
> exclusive access means a write, so the folio better be dirty (unless we
> aborted halfway through). But then I couldn't find the code in nouveau to
> do that, so now I'm confused.

That confused me as well. Requiring the PTE to be writable does not 
imply that it is dirty.

So something must either set the PTE or the folio dirty.

( In practice, most anonymous folios are dirty most of the time ... )

If we assume that "device-exclusive entries" are always dirty, then it 
doesn't make sense to set the folio dirty when creating device-exclusive 
entries. We'd always have to set the PTE dirty when restoring the 
exclusive pte.

-- 
Cheers,

David / dhildenb