linux-kernel - Re: [PATCH v1 07/12] mm/page_vma_mapped: device-private entries are not migration entries

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Message-ID: <baraw2czilsofpedok3jtxxshfaxjrmef6u42pitibtcst6mel@scoukpiwnvvc>
Date: Fri, 31 Jan 2025 10:36:23 +1100
From: Alistair Popple <apopple@...dia.com>
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>, Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1 07/12] mm/page_vma_mapped: device-private entries are
 not migration entries

On Wed, Jan 29, 2025 at 12:54:05PM +0100, David Hildenbrand wrote:
> It's unclear why they would be considered migration entries; they are
> not.

Yeah, I agree that doesn't seem right. I suspect I was initially modelling
device exclusive entries similar to migration entries but obviously went too
far. So thanks for fixing:

Reviewed-by: Alistair Popple <apopple@...dia.com>

> Likely we'll never really trigger that case in practice, because
> migration (including folio split) of a folio that has device-private
> entries is never started, as we would detect "additional references":
> device-private entries adjust the mapcount, but not the refcount.
> 
> Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/page_vma_mapped.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 81839a9e74f1..32679be22d30 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -111,8 +111,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
>  			return false;
>  		entry = pte_to_swp_entry(ptent);
>  
> -		if (!is_migration_entry(entry) &&
> -		    !is_device_exclusive_entry(entry))
> +		if (!is_migration_entry(entry))
>  			return false;
>  
>  		pfn = swp_offset_pfn(entry);
> -- 
> 2.48.1
>