linux-kernel - Re: [PATCH v1 2/6] fs/proc/task_mmu: don't indicate PM_MMAP_EXCLUSIVE without PM

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Date: Mon, 10 Jun 2024 06:38:33 +0200
From: Oscar Salvador <osalvador@...e.de>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v1 2/6] fs/proc/task_mmu: don't indicate
 PM_MMAP_EXCLUSIVE without PM_PRESENT

On Fri, Jun 07, 2024 at 02:23:53PM +0200, David Hildenbrand wrote:
> Relying on the mapcount for non-present PTEs that reference pages
> doesn't make any sense: they are not accounted in the mapcount, so
> page_mapcount() == 1 won't return the result we actually want to know.
> 
> While we don't check the mapcount for migration entries already, we
> could end up checking it for swap, hwpoison, device exclusive, ...
> entries, which we really shouldn't.
> 
> There is one exception: device private entries, which we consider
> fake-present (e.g., incremented the mapcount). But we won't care about
> that for now for PM_MMAP_EXCLUSIVE, because indicating PM_SWAP for them
> although they are fake-present already sounds suspiciously wrong.
> 
> Let's never indicate PM_MMAP_EXCLUSIVE without PM_PRESENT.

Alternatively we could use is_pfn_swap_entry?
But the PM_PRESENT approach seems more correct.

> Signed-off-by: David Hildenbrand <david@...hat.com>

Signed-off-by: Oscar Salvador <osalvador@...e.de>

-- 
Oscar Salvador
SUSE Labs