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Sync RWP delivers a message and blocks the faulting thread until the handler resolves the fault. For working-set tracking the VMM does not need the message: it just needs to know, at scan time, which pages were touched. Async RWP serves that use case — the kernel restores access in-place and the faulting thread continues without blocking. The VMM reconstructs the access pattern after the fact via PAGEMAP_SCAN: pages whose uffd bit is still set (inverted PAGE_IS_ACCESSED) were not re-accessed since the last RWP cycle. Worth calling out: async resolution upgrades writable private anon PTEs via pte_mkwrite() when can_change_pte_writable() allows, mirroring do_numa_page(). Without it, every re-access of an RWP'd writable page would COW-fault a second time. UFFD_FEATURE_RWP_ASYNC requires UFFD_FEATURE_RWP. Link: https://lore.kernel.org/20260708111417.173443-13-kirill@shutemov.name Signed-off-by: Kiryl Shutsemau <kas@kernel.org> Assisted-by: Claude:claude-opus-4-6 Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: David Hildenbrand <david@kernel.org> Cc: James Houghton <jthoughton@google.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Liam Howlett <liam@infradead.org> Cc: Lorenzo Stoakes <ljs@kernel.org> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Peter Xu <peterx@redhat.com> Cc: Sean Christopherson <seanjc@google.com> Cc: SeongJae Park <sj@kernel.org> Cc: Suren Baghdasaryan <surenb@google.com> Cc: Vlastimil Babka <vbabka@kernel.org> Cc: Zi Yan <ziy@nvidia.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>