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 gR9XVNAF 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025-02-28 17:32, Peter Xu wrote: > 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. 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