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[98.225.44.182]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2ccc9bf74cbsm63356135ad.18.2026.07.10.11.07.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Jul 2026 11:07:26 -0700 (PDT) Date: Fri, 10 Jul 2026 11:07:23 -0700 From: Stanislav Kinsburskii To: Jason Gunthorpe Cc: airlied@gmail.com, akhilesh@ee.iitb.ac.in, akpm@linux-foundation.org, corbet@lwn.net, dakr@kernel.org, david@kernel.org, decui@microsoft.com, haiyangz@microsoft.com, kees@kernel.org, kys@microsoft.com, leon@kernel.org, liam@infradead.org, lizhi.hou@amd.com, ljs@kernel.org, longli@microsoft.com, lyude@redhat.com, maarten.lankhorst@linux.intel.com, mamin506@gmail.com, mhocko@suse.com, mripard@kernel.org, nouveau@lists.freedesktop.org, ogabbay@kernel.org, oleg@redhat.com, rppt@kernel.org, shuah@kernel.org, simona@ffwll.ch, skhan@linuxfoundation.org, surenb@google.com, tzimmermann@suse.de, vbabka@kernel.org, wei.liu@kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-rdma@vger.kernel.org Subject: Re: [PATCH v7 2/8] mm/hmm: add hmm_range_fault_unlocked_timeout() for mmap lock-drop support Message-ID: References: <178345345668.660027.2952911919681614557.stgit@skinsburskii> <178345362182.660027.12809852179204464964.stgit@skinsburskii> <20260710162749.GP118978@ziepe.ca> <20260710164743.GX118978@ziepe.ca> <20260710180312.GD1803712@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260710180312.GD1803712@ziepe.ca> X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 5B2651C0002 X-Stat-Signature: 4wt9sjyqre7xfge4jt5aqwkdtp46o7o5 X-HE-Tag: 1783706848-512615 X-HE-Meta: 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 oOS0T1Sj 7w/iT1xD1kd2LgYxjE6r6klY4FwUlyK2LzfUGbRnnzGGEX6MHqSm6igESgJqFzvW3RSpLoTTRiTH/p1ak00teM11+mN3zpJSkQzZ10PYLvlu7Hab5uW04V8AyARy+MTjrMv3zXiq8AGN8VrdVO+iB7a3W5+HQqMfcPTY60NXQDkrX2JvEwHpddLmDiG/7sjNqL0wMyfZwTjwiWuwUZqDdhBASoso/PB2GR4PMF0fhvXJxHchiCgGQDxEgauhPkxRQ424mebXiZMlBJNlACmgrz3qPL0koibQCskgAoVlMpEGMrFJS4GP0ympgpQTW95kpi1GnWckTtHhjNH+9KNCTgy4tBE9E9WQyCrWpZ74/RfjeVKmwmt8VbI5VJ+f5FF/WDV7bmo+7Xm4uE37/81zvpQJVr6LBZxBSaGtPdh6kEOWj41smW8M1sRL5GPTetBKi/ot/xYxBhP9bZEAiMvsrpF7rlpoRoaFyayC832z9mVoe+iGbvhlYvmF7cNOkdJN6k3LDae0p2boNcIPFyjQfVqvTuw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jul 10, 2026 at 03:03:12PM -0300, Jason Gunthorpe wrote: > On Fri, Jul 10, 2026 at 10:02:55AM -0700, Stanislav Kinsburskii wrote: > > The main customer for this new feature I have in mind is the MSHV driver > > which backs VMs memory with HMM, requires userfaultfd support for > > post-copy live migration and fast restore and it doesn't timeout. > > > > I agree, that this current timeout value used by the other callers might > > not be enough to repopulate the mappings with userfaultfd, but there > > drivers would get -EFAULT for uderfaultfd-backed mappings without this > > change anyway, so getting -EBUSY with the change instead doesn't look > > like a significant change to the behaviour from my POV. > > It sounds like it won't be reliable either then. > > > > So, maybe the deadline should be resetting after every handled fault? > > > ie the timeout really is only about the mmu notifier and we don't > > > count the time spent handling faults or walking? > > > > The timeout was inherited from existing HMM users rather than introduced > > as a new HMM policy. Some GPU drivers use HMM_RANGE_DEFAULT_TIMEOUT as a > > budget for the whole range population operation, including HMM retries > > and subsequent driver mapping work. > > Yes, because we always had a timeout around the notifier because that > scheme can sort of live lock. The timeout was to protect that only, ie > limit the number of notifier retries. > > Expanding the timeout to be outside what is bounded by the notifier > retry is not right, and heavy stuff like mapping should be done after > the hmm side succeeds and the notifiers concluded so they can rely on > normal locking instead. > > This is why I'm suggesting to reset the deadline as hmm makes forward > progress, we really only want to bound the notifier retry loop not > anything else. > Sure, I'll modify accordingly. Thanks, Stanislav > Jason