Re: [PATCH 05/12] fs/proc/task_mmu: write-protect device-private THPs in PAGEMAP_SCAN

[Gregory Price (Meta)]

On 2026-11-11 12:55 -0500, Tal Zussman wrote:
> PAGEMAP_SCAN with PM_SCAN_WP_MATCHING write-protects a huge PMD through
> make_uffd_wp_pmd(), which handles a present THP and a migration entry
> but does nothing for a device-private THP. The PMD never gets the uffd
> bit.
> 
> pagemap_thp_category() reports a non-present PMD without the uffd bit as
> written. A user tracking writes with async uffd-wp therefore sees every
> page of a device-private THP as written on every scan, whether it was
> written to or not. This requires a driver that migrates THPs to device
> memory.
> 
> A device-private PMD must be write-protected like any other huge PMD. To
> fix it, pass it to set_uffd_wp_swp_pmd() as well, which downgrades a
> writable entry and sets the uffd bit. When the THP migrates back,
> set_pmd_migration_entry() carries the uffd bit over to the migration
> entry, so the write protection is kept.
> 
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>

reproduced the issue by observing we fail to carry back the write
protect bit after migration.

Implementation details i'll leave to Lorenzo's feedback, i don't feel
strongly about it, it's gonna look bad either way i think :\

Tested-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>

-- 
~Gregory