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: <z7s7xb2ftv5hqg3uzjqkou6enguleazwwehxbi5zulbkar2aej@zlbdh2kdewdn>
Date: Fri, 31 Jan 2025 10:06:53 +1100
From: Alistair Popple <apopple@...dia.com>
To: David Hildenbrand <david@...hat.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 05/12] mm/memory: detect writability in
 restore_exclusive_pte() through can_change_pte_writable()

On Thu, Jan 30, 2025 at 02:03:42PM +0100, Simona Vetter wrote:
> On Thu, Jan 30, 2025 at 10:58:51AM +0100, David Hildenbrand wrote:
> > 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.
> 
> Yeah I'm not finding that something.
> 
> > ( In practice, most anonymous folios are dirty most of the time ... )
> 
> And yup that's why I think it hasn't blown up yet.
> 
> > 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.
> 
> I do agree with your change, I think it's correct to put this
> responsibility onto drivers. It's just that nouveau seems to not be
> entirely correct.

Yeah, agree it should be a driver responsibility but also can't see how nouveau
is correct there either. I might see if I can get it to blow up...

> And thinking about this I have vague memories that I've discussed the case
> of the missing folio_set_dirty in noveau hmm code before, maybe with
> Alistair. But quick search in archives didn't turn up anything.

I have vague recollections of that, but I could be confusing it with some of the
migrate_vma_*() issues we had dropping dirty bits (see
https://lkml.kernel.org/r/dd48e4882ce859c295c1a77612f66d198b0403f9.1662078528.git-series.apopple@nvidia.com)

> -Sima
> -- 
> Simona Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch