linux-kernel - Re: [PATCH v1] fs/proc/task_mmu: Fix uffd-wp confusion in pagemap_scan_pmd

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Message-ID: <cfc6b006-ef6f-4099-b29a-fba30bdbd74d@redhat.com>
Date: Mon, 29 Apr 2024 14:22:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muhammad Usama Anjum <usama.anjum@...labora.com>,
 Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v1] fs/proc/task_mmu: Fix uffd-wp confusion in
 pagemap_scan_pmd_entry()

On 29.04.24 13:41, Ryan Roberts wrote:
> pagemap_scan_pmd_entry() checks if uffd-wp is set on each pte to avoid
> unnecessary if set. However it was previously checking with
> `pte_uffd_wp(ptep_get(pte))` without first confirming that the pte was
> present. It is only valid to call pte_uffd_wp() for present ptes. For
> swap ptes, pte_swp_uffd_wp() must be called because the uffd-wp bit may
> be kept in a different position, depending on the arch.
> 
> This was leading to test failures in the pagemap_ioctl mm selftest, when
> bringing up uffd-wp support on arm64 due to incorrectly interpretting
> the uffd-wp status of migration entries.
> 
> Let's fix this by using the correct check based on pte_present(). While
> we are at it, let's pass the pte to make_uffd_wp_pte() to avoid the
> pointless extra ptep_get() which can't be optimized out due to
> READ_ONCE() on many arches.
> 
> Closes: https://lore.kernel.org/linux-arm-kernel/ZiuyGXt0XWwRgFh9@x1n/
> Fixes: 12f6b01a0bcb ("fs/proc/task_mmu: add fast paths to get/clear PAGE_IS_WRITTEN flag")
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---

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

-- 
Cheers,

David / dhildenb