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Reed" , Andrew Morton , linux-mm@kvack.org, Axel Rasmussen Subject: Re: PROBLEM: userfaultfd REGISTER minor mode on MAP_PRIVATE range fails Message-ID: References: <1757967196.153116687@apps.rackspace.com> <1757977128.137610687@apps.rackspace.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: KC6Z4W-V9dhzoptY9gqo1C9Gb6hm7ANLA1wjuF20jqY_1758034101 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: jyoie5cxjesz9yqig4gxkdc9ijch9ixx X-Rspam-User: X-Rspamd-Queue-Id: F3C3818000F X-Rspamd-Server: rspam04 X-HE-Tag: 1758034103-411377 X-HE-Meta: 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 odPewOB3 BnsGU+p8izpRmrSyOA6iGpCjBwbb8IPS4SZpnzuJ0OK4syCObhtgWG+LV4qJ3x9TZd842xt8RcCr8D8n7dkP9CfpsUPKdhi1ZDc84skGxymP1JIJL160XsHZZ/5K9yoOCungs4Flw6m/wne/M3BTy8+263qmx92I/QdCMI9lsEmLYBi7fVzBtnOLbChsJlUqWvdqj34LCnkne4/cFZzu811obE6sLmClOtVEo0cigAGM39rrlaTXdQrTVe9UelKX/Rd8p/pC93PoRniiF9K05ctBPEOeu73LIvlXG7ZN6NMt1AmilEcju41CGXOS0KCQys47Tfyyb1goYJpyKhk3warS70Q== 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 Mon, Sep 15, 2025 at 05:31:51PM -0700, James Houghton wrote: > I still don't have a solid grasp of what your use case is. David, are you trying to provide a synchronous trap for an anon swapin event? Say, you want to be able to stop a thread from swapping in anything from disk (or swap cache), do something, and UFFDIO_CONTINUE to kick it off again? That might make some sense when trying to match what MINOR mode means v.s. the mm's minor faults, but some explanation of why you wanted to do that would be helpful. I agree with James that it was at least not the intention when userfaultfd MINOR trap was introduced. The other thing to mention is, AFAIU userfaultfd's major use case is not through a fork(), even though it should work.. In many cases, userfaultfd is used within a process, with a dedicated thread resolving faults. When it's used across processes, fork() should work but UFFD_FEATURE_EVENT_FORK is required, or otherwise via SCM_RIGHTS. For the latter, the tracee needs to not only share the uffd object, but tell the tracer explicitly about the memory layout, because those addresses in the events will be reported in tracee's mm address space. Thanks, -- Peter Xu