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

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Message-ID: <20250130140832.GM5556@nvidia.com>
Date: Thu, 30 Jan 2025 10:08:32 -0400
From: Jason Gunthorpe <jgg@...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>, 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 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.

Jason