On Fri, May 08, 2026 at 04:55:23PM +0100, Kiryl Shutsemau (Meta) wrote: > 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. > > Signed-off-by: Kiryl Shutsemau <[email protected]> > Assisted-by: Claude:claude-opus-4-6
Acked-by: Mike Rapoport (Microsoft) <[email protected]> > --- > fs/userfaultfd.c | 19 ++++++++++++++++++- > include/linux/userfaultfd_k.h | 6 ++++++ > include/uapi/linux/userfaultfd.h | 11 ++++++++++- > mm/huge_memory.c | 25 ++++++++++++++++++++++++- > mm/hugetlb.c | 32 +++++++++++++++++++++++++++++++- > mm/memory.c | 27 +++++++++++++++++++++++++-- > 6 files changed, 114 insertions(+), 6 deletions(-) -- Sincerely yours, Mike.
