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Message-ID: <Z5ukdq6LG2dQKaPU@phenom.ffwll.local>
Date: Thu, 30 Jan 2025 17:10:30 +0100
From: Simona Vetter <simona.vetter@...ll.ch>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: 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>, Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH v1 08/12] mm/rmap: handle device-exclusive entries
correctly in try_to_unmap_one()
On Thu, Jan 30, 2025 at 10:08:32AM -0400, Jason Gunthorpe wrote:
> On Thu, Jan 30, 2025 at 02:06:12PM +0100, 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'm not sure it is meaningful, but in principle a driver could keep
> track of the poisoned private memory using that struct page
> bit. Perhaps in that sense it is more of a driver private flag than
> something the core MM would touch.
>
> If you have a coherent interconnect then you should not be using
> device private.
Yes on both, that's what I meant.
-Sima
--
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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