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See UFFDIO_REGISTER_MODE_WP: >>> >>> https://man7.org/linux/man-pages/man2/userfaultfd.2.html >> >> userfaultfd UFFDIO_REGISTER only seems to work if I pass an address >> resulting from a mmap mapping, but returns EINVAL if I pass a >> page-aligned address which sits within a private file mapping >> (e.g. executable data). > > Yes, so far sync traps only supports RAM-based file systems, or anonymous. > Generic private file mappings (that stores executables and libraries) are > not yet supported. OK, this confirms my observations. > >> >> Also, I notice that do_wp_page() only calls handle_userfault >> VM_UFFD_WP when vm_fault flags does not have FAULT_FLAG_UNSHARE >> set. > > AFAICT that's expected, unshare should only be set on reads, never writes. > So uffd-wp shouldn't trap any of those. I'm confused by your comment. I thought unshare only applies to *write* faults. What am I missing ? > >> >> AFAIU, as it stands now userfaultfd would not help tracking COW faults >> caused by stores to private file mappings. Am I missing something ? > > I think you're right. So we have UFFD_FEATURE_WP_ASYNC that should work on > most mappings. That one is async, though, so more like soft-dirty. It > might be doable to try making it sync too without a lot of changes based on > how async tracking works. I'll try this out. It may not matter that it's async given a use-case use-cases of tracking the age since the WP fault on the COW pages. We don't need to react to the event in-place to alter its behavior, just a notification should be fine AFAIU. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com