On Fri, May 22, 2026 at 02:38:55PM +0100, Kiryl Shutsemau wrote: > From: "Kiryl Shutsemau (Meta)" <[email protected]> > > Add an admin-guide section covering UFFDIO_REGISTER_MODE_RWP: > > - sync and async fault models; > - UFFDIO_RWPROTECT semantics; > - UFFD_FEATURE_RWP_ASYNC; > - UFFDIO_SET_MODE runtime mode flips. > > It also covers typical VMM working-set-tracking workflow from detection > loop through sync-mode eviction and back to async. > > Signed-off-by: Kiryl Shutsemau <[email protected]> > Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Mike Rapoport (Microsoft) <[email protected]> with some nits below > --- > Documentation/admin-guide/mm/userfaultfd.rst | 226 ++++++++++++++++++- > 1 file changed, 220 insertions(+), 6 deletions(-) > > diff --git a/Documentation/admin-guide/mm/userfaultfd.rst > b/Documentation/admin-guide/mm/userfaultfd.rst > index 1e533639fd50..cb5d0e0c9fff 100644 > --- a/Documentation/admin-guide/mm/userfaultfd.rst > +++ b/Documentation/admin-guide/mm/userfaultfd.rst ... > The user app can collect the "written/dirty" status by looking up the > -uffd-wp bit for the pages being interested in /proc/pagemap. > +uffd bit for the pages being interested in /proc/pagemap. It's pre-existing, but "being interested" sounds weird to me Maybe let's change "being interested" to "of interest" or "the app is interested in". > -The page will not be under track of uffd-wp async mode until the page is > +The page will not be under track of userfaultfd-wp async mode until the page > is Here as well I'd s/will not be under track of/will not be tracked by/ > explicitly write-protected by ``ioctl(UFFDIO_WRITEPROTECT)`` with the mode > flag ``UFFDIO_WRITEPROTECT_MODE_WP`` set. Trying to resolve a page fault > that was tracked by async mode userfaultfd-wp is invalid. > @@ -307,6 +307,220 @@ transparent to the guest, we want that same address > range to act as if it was > still poisoned, even though it's on a new physical host which ostensibly > doesn't have a memory error in the exact same spot. > > +Read-Write Protection > +--------------------- ... > +**Setup:** > + > +1. Open a userfaultfd and enable ``UFFD_FEATURE_RWP`` via ``UFFDIO_API``. > + Optionally request ``UFFD_FEATURE_RWP_ASYNC`` as well — it requires > + ``UFFD_FEATURE_RWP`` to be set in the same ``UFFDIO_API`` call. > + > +2. Register the guest memory range with ``UFFDIO_REGISTER_MODE_RWP`` > + (and ``UFFDIO_REGISTER_MODE_MISSING`` if evicted pages will need to be > + fetched back from storage). I'd make it 2. Register the guest memory range with ``UFFDIO_REGISTER_MODE_RWP``. Add ``UFFDIO_REGISTER_MODE_MISSING`` if evicted pages will need to be fetched back from storage. -- Sincerely yours, Mike.
