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[89.27.86.246]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5b8d46e0966sm191742e87.55.2026.09.21.22.35.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 21 Sep 2026 22:35:15 -0700 (PDT) From: mpenttil@redhat.com To: linux-mm@kvack.org Cc: dri-devel@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-kernel@vger.kernel.org, =?UTF-8?q?Mika=20Penttil=C3=A4?= , David Hildenbrand , Jason Gunthorpe , Leon Romanovsky , Alistair Popple , Balbir Singh , Zi Yan , Matthew Brost , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko Subject: [PATCH v14 12/12] Documentation/mm/hmm: document migration through hmm_range_fault() Date: Tue, 22 Sep 2026 08:34:21 +0300 Message-ID: <20260922053421.4092027-13-mpenttil@redhat.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260922053421.4092027-1-mpenttil@redhat.com> References: <20260922053421.4092027-1-mpenttil@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: o7j37jrGKtBjCYAI-ESrs32I2cDhjRs0NuR0TxOzFSE_1790055319 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 913F918000D X-Stat-Signature: 9r1w851gssofqxj34octqzcuo6q4p3no X-HE-Tag: 1790055322-133927 X-HE-Meta: U2FsdGVkX19FNG2sAZ/8PaM+4FbiqVyvxY1JYzIsJqMB1qNU6uBdCQ8+13Wtdz4UYyRSeT5did4bqytb9JJaW+j5Xz5CS8xAm5nQ2aw1MtDS1JU0zLWXkfU++lNMH7uOPUS8RkSAJeYnPidorIhyOGaWyaEO40K49Tbme2lnVOWeXUyzIp98JSo/e7K9BeLzeSrbibw/EYNC5e0Ir/YfRiNSOFqi/+p6ucca7JL3nwNLQGfFTVIrY1h9YrJzNeNkKIFkzwYUVk46+szOizz9ImIBDv9FN0WGM8RkY6N/zSkcX/Pt6xso+UxnUx9nqdN/bKhRav0T9OzaFcm7BcrwaN7hew/PiWUdABedVlDMPQkpIBFsBRGuUo46/3ZmwHnoyti19tdMuWVQ32KAQlCjrSB6j/yY46k7kqMaqm+ZmYxW73eygp1jnU3g5ddc0BMmgZhUnTfRm0O6iyUNh3zewOFjzfBH4qcb41uvGJIOz5hHrjpGz/CSuHBwiIRjagMFeffXfijWfgVx8WzQVs8pyJS9WJemMSoU2yqsBm5YQhyVeMTnX2n0uJd49jEyD06yukJL1K/5sMaCj5Kc4aANx34KHHW760d6iIzIowc5buAiGEzD2oa/FOkLXM/bsZPoyVyJ0DSVZUVwJdVDXc+ANpE5E++dgPoAIxq0wb6Pcm36AtHyOQhg+SXyWPR01/Ug5NlA9fRtasrh2xwKK+AQ0i68toi4d1eXAK5MgjcxSd5C0d1X7Qsa+wtw8VHQJQk1ax07Mpqi0Qg4GUQqAY5V8kmiQlPfxNNgTHqFqZ09bEzH5JFuOw6Vz+mbva0H6IvotzvbF/YA6LIAHms1e7DdDNL/1orQWz4b2Dx8mJtUveQugsrSIy3ndO3Bs4xxL8iBHP5gawNjmQsfQ4UkrzETcIvnJuvesUftZ3zLGNyI2HB3Z8Q8FKYBEyf8EFigDAmzH21Uk/CjCcI9blDm+eL OZQZie23 RatwhB6ON5/zhwblWvEaf3aO+f0tPUq7Yx1FRB/sf9AHfi1P1DLQvvMO5bjF3+y1y2A/RJpn97E+8UnN6vOZ2QDJfnsoDhNaMG9C0cGkTsxFLyFRLma/PGDZCQsws3fNIapK+4vg3smorO8f8SYnQB2XkFzSlEvQrUyBQ5JwTUY3vszeRGqCZqpaLlhDn/RUfAboUyfiXB2o2yC3XKYhF9ocFO6FbgMSGzotO2Q7OCrBpO17KhHr3wL94dTA3gv2aGJb2km5hS2rS7Jftf85mC2rT2uMX/Yy/J6BWVGvfV5eHjdMjVDuSKbEdCRkMkhpClqJYCTCtpy6EAZ3TNwpV7+/NoQJNrJfa58yVEBXxPqkyz3l5P//h4CM0ZqeGFBf8XgssUrKTxJ030DIqAbFK2SwGQexWvFm4Nqo/FEcPU97kViV40e5hlAbibyZmlHWQYq0Nx0BFoaCYRkd+IVltPgDURA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Mika Penttilä Describe the two new MIGRATE_VMA_FAULT / MIGRATE_VMA_WRITE flags of migrate_vma_setup(), and add a section on driving the migration collection phase directly from hmm_range_fault() via HMM_PFN_REQ_MIGRATE and migrate_hmm_range_setup(). Signed-off-by: Mika Penttilä --- Documentation/mm/hmm.rst | 39 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) diff --git a/Documentation/mm/hmm.rst b/Documentation/mm/hmm.rst index fc1b8dc19825..58424e5f5873 100644 --- a/Documentation/mm/hmm.rst +++ b/Documentation/mm/hmm.rst @@ -348,6 +348,13 @@ between device driver specific code and shared common code: Currently only anonymous private VMA ranges can be migrated to or from system memory and device private memory. + By default only pages already present are collected. Two additional flags + ask migrate_vma_setup() to fault in missing pages first: + + * ``MIGRATE_VMA_FAULT`` faults in missing pages with read access. + * ``MIGRATE_VMA_WRITE`` faults in missing pages with write access + (implies faulting). + One of the first steps migrate_vma_setup() does is to invalidate other device's MMUs with the ``mmu_notifier_invalidate_range_start()`` and ``mmu_notifier_invalidate_range_end()`` calls around the page table @@ -427,6 +434,38 @@ between device driver specific code and shared common code: The lock can now be released. +Migration collection through hmm_range_fault() +============================================== + +The collection phase of migration (steps 1 and 2 above) can also be driven by +hmm_range_fault() directly, sharing its page table walk. This lets a driver +fault in and collect a range for migration in one walk, which is useful for +migrate on fault. + +To do so, the driver sets ``HMM_PFN_REQ_MIGRATE`` in ``range->default_flags`` +and points ``range->migrate`` at a ``struct migrate_vma`` it has filled in +(``flags``, ``src``, ``dst``, ``pgmap_owner``). Usually ``HMM_PFN_REQ_FAULT`` +(and ``HMM_PFN_REQ_WRITE``) is set as well, so missing pages are faulted in +before being collected. The mmap_read_lock() has to be held for the whole +migration, since the vma must stay stable. + +hmm_range_fault() collects the entries the same way migrate_vma_setup() does, +taking a folio reference, locking it and installing a migration PTE. Collected +entries are marked with ``HMM_PFN_VALID | HMM_PFN_MIGRATE`` in +``range->hmm_pfns``. If the page tables change while locks are dropped the +partially collected entries are rolled back automatically. + +After hmm_range_fault() returns, the driver calls:: + + void migrate_hmm_range_setup(struct hmm_range *range); + +to translate ``range->hmm_pfns`` into ``migrate->src[]`` (``migrate->dst[]`` is +zeroed) and initialize ``migrate->cpages`` and ``migrate->npages``. From here on +the ``range->migrate`` struct is ready for the rest of the flow (steps 3 +onwards), i.e. migrate_vma_pages() and migrate_vma_finalize(). This should be +called even on error, since hmm_range_fault() may have collected part of the +range before failing. + Exclusive access memory ======================= -- 2.55.0