On Wed, Mar 13, 2024 at 11:03:04PM +0100, David Hildenbrand wrote: > On 13.03.24 22:31, peterx@xxxxxxxxxx wrote: > > From: Peter Xu <peterx@xxxxxxxxxx> > > > > Commit 0cf18e839f64 of large folio zap work broke uffd-wp. Now mm's uffd > > unit test "wp-unpopulated" will trigger this WARN_ON_ONCE(). > > Good that I added the WARN_ON_ONCE() :) To be explict, VM_WARN_ON_ONCE. :) And that's my guess that you didn't hit it when you posted the series and did the tests, as I know latest distros like Fedora dropped DEBUG_VM, so maybe you had your base config out of there (but I normally have it irrelevant of that). > > > > > The WARN_ON_ONCE() asserts that an VMA cannot be registered with > > userfaultfd-wp if it contains a !normal page, but it's actually possible. > > One example is an anonymous vma, register with uffd-wp, read anything will > > install a zero page. Then when zap on it, this should trigger. > > Are you sure? zap_install_uffd_wp_if_needed() contains right at the start: > > /* Zap on anonymous always means dropping everything */ > if (vma_is_anonymous(vma)) > return; My example is not exactly how the test failed, but should be a simpler version of it. To trigger this warning I don't think it requires the zero page to be wr-protected at all or have any pte marker involved. UFFDIO_REGISTER should suffice, afaiu (feel free to read the example above again; there's no mention of ioctl(UFFDIO_WRITEPROTECT)). > > So if that's the case the unit test triggers, I'm confused. > > > > > What's more, removing that WARN_ON_ONCE may not be enough either, because > > we should also not rely on "whether it's a normal page" to decide whether > > pte marker is needed. For example, one can register wr-protect over some > > DAX regions to track writes when UFFD_FEATURE_WP_ASYNC enabled, in which > > case it can have page==NULL for a devmap but we may want to keep the marker > > around. > > I thought uffd-wp was limited to specific backends only. But looks like that > changed with UFFD_FEATURE_WP_ASYNC, I guess? Correct. That was also what the new PAGEMAP ioctl relies on. > > > Change itself looks, good. Not sure about the anon_vma example above. > > Thanks! > > Acked-by: David Hildenbrand <david@xxxxxxxxxx> Thanks, -- Peter Xu
