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charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 03438C000A X-Stat-Signature: f8d95c8hm8moae875ime51qmspfzq75x X-HE-Tag: 1740781970-465414 X-HE-Meta: U2FsdGVkX18ZAvlaO0VWdA2KMppaOfz9GBOXbU/30gaoEnS0J2ZgdzObmGKUaxQvC9UPSq9vdQt+8O8P6pN4vpqzNoZD76J44Im00SM3k7ELW+ub5x+RDsvTxBted4Rwazwr3+3PTpsHx9XmoBy6fF3b7aeK7eQ0nb4PJZVJFvdpi7K5lZseZya+uJUpji5MISXSiWOZ5knPKHdxBwRDlDsnnKWLrCcUG77B3CJ/bId3UvcQZWDAB2dnyFyKRO/xS3ArchUisVhtixV/vVy2RsVuIzF0rjYg2fSG7moT66yHHWHdXrRFOHoaXDQBs4YIBAuHTvGKskZh5gUZgHJaV8nXl1GkweYASVF75w7baRu3Q+q1d8NYIxPYxwKc89dSMVKrZqEzQECe5jA2+UGMhRrwgijAX1f0H0MaHwtfulQsYKo6A0pPsqAFenCyOy0YLCShhuQSbvhoO8507ZGH9Jo9wrov4WRl/PDPMwcitsG0/+08KqD/piv3CqFk5p668jgkdgxt0DLFoE72628JEUnc9LA/TWHJGh+NaeUFe8S8LweaQ1Sj03gj0rmJiny1Imb+H9vsC/ZlfTdQy64I56uy94m1CTvK7FCwXqdc2pKG0PBmgR33a7rm7YSqEU8QLRFA1l/yMPLLCxO9wI5EDQbMunmGnYBkUXQSo8L/qfOKk7CPjchn6Iy/En3D7Vi7QE1nTMlAu6AYW1MgkhWrodOSf6qhK75LcdsHFBw4dZyLnPl6twCvhhbORJs9K9ROL5OJR7zKw/8l6XiQoSY9/ne3NoSUegSHVMNWk+oBbYJQNa5bWoY5e7ppIpiGcZMx73etA0KcBxzcVRwj+Ox6BFYStFl51TB75rVc2ox63EpDAYR8dWOxPsu0X306jJi4ZpENaOCrBd0TQoI0bmCGGQngtTKe9HuNVkNKSn10AiDS+0/hLzLDvY3ECbnriPhc2ljNmUuKxXJ6hawuxNn xR4HWJHv juPiQf3oSP9fL7/3Hr13SeTpsapVg5EnIgI6KA6JMrLavJytrNPv9qZmN2f7d+qWSutMuAHrmcov28IJI2+V59ThGaaoBAWsKoBaJDsSK3N+wxlizlh0JXc+bI/j+8lIZkQPKJTGXBDw0k36ClfqCO2aHuKngLRU4I1CSbCknA8Jgr99VupkZ5370ge5+a6p/I0V/ZkXtRCIYhjJcvY/GLL+788hH4SP3b0oB+ASYaSmNltEQ0jy7XHiRVx7T083Wi21XcEfSsd5iSnH6gmrzCqb2WzUjznRR7ZP3auJYaLJtv7JiYr3LR9TgnkLD1xBODpi/3sxbDBD2LqCdaWMKsQ2oU4CWLKSxV7t6FsTHU/x7NPIwlOlX0ggeWstHVuJo0t9XM+sjYhxQwIIinBY/vnsvAJiBLQSaHVCH 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 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