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 06:49:09 +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.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Forgot to comment on something:

> @@ -1517,14 +1514,13 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  			if (pmd_swp_uffd_wp(pmd))
>  				flags |= PM_UFFD_WP;
>  			VM_BUG_ON(!is_pmd_migration_entry(pmd));
> -			migration = is_migration_entry(entry);
>  			page = pfn_swap_entry_to_page(entry);

We do not really need to get the page anymore here as that is the non-present
part.

Then we could get away without checking the flags as only page != NULL
would mean a present pmd.

Not that we gain much as this is far from being a hot-path, but just
saying..

-- 
Oscar Salvador
SUSE Labs