On Fri, Feb 28, 2025 at 12:53:02PM -0500, Mathieu Desnoyers wrote: > On 2025-02-28 11:32, Peter Xu wrote: > > On Fri, Feb 28, 2025 at 09:59:00AM -0500, Mathieu Desnoyers wrote: > > > For the VM use-case, I wonder if we could just add a userfaultfd > > > "COW" event that would notify userspace when a COW happens ? > > > > I don't know what's the best for KSM and how well this will work, but we > > have such event for years.. 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. > > 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. > > 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. Thanks, -- Peter Xu
