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

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Message-ID: <Z5t5RKFwl34vpqU4@phenom.ffwll.local>
Date: Thu, 30 Jan 2025 14:06:12 +0100
From: Simona Vetter <simona.vetter@...ll.ch>
To: David Hildenbrand <david@...hat.com>
Cc: 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 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.

Also to clarify, I meant this as a general comment for all subsequent
patches that have the same paragraph.
-Sima
-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch