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b=RXe5/6ccLygZW7M8ERjzrGvEej+jrQdq6ny54rod4vAmYz2OQqbcb/EKy0q5fC/v/ e17Uvg16pRJrAeyKJ8NhtBIC8s2Rkkip407p/Bp5oJd1Gp1AcgRHoSbLd/HQO7Cvp7 SE5fTt6PDfxYsIihDlUBVIs6FP0TQpDgViMCS2hIk1ZhybI1v7cs/tDNm07jBBN201 0cVx6iqEqpRvgahrMOIMUF1aHhnrAjWdzUmvNT8gHOMK+8U00SFQDRGfRxzfrzTwYB a9Azh5+Yp4oxVMXGM4gTwyhFB+mnWgFo/QHeep6CojMG+HPRK/EklnyD9jaZTxxn/D JWgEe1I5S0b0Q== From: "Barry Song (Xiaomi)" To: akpm@linux-foundation.org, linux-mm@kvack.org Cc: x86@kernel.org, linux-arm-kernel@lists.infradead.org, surenb@google.com, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, shakeel.butt@linux.dev, david@kernel.org, linux-kernel@vger.kernel.org, zhanghongru@xiaomi.com, willy@infradead.org, zhangbo56@xiaomi.com, "Barry Song (Xiaomi)" Subject: [RFC PATCH v1 0/2] mm: use VMA lock for kernel faults on user addresses Date: Sun, 2 Aug 2026 15:40:16 +0800 Message-Id: <20260802074018.73887-1-baohua@kernel.org> X-Mailer: git-send-email 2.39.3 (Apple Git-146) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: A94B510000B X-Stat-Signature: 5wiaswngogignqg1s1aw3mrhjmr99wd3 X-Rspam-User: X-HE-Tag: 1785656433-288210 X-HE-Meta: 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 2Z71n/6Q +siCpPtk+wb1zJLCLa4IrxC5HY6+tB/liRINBDRUPN+rMGQRLSqCrUlWQpVJKAtcpoLAr/W85C52XJ3K0OHUUWrKOkj4XE1nsJP8bHcEj2CNd2PuklbOQFtlLi4i18ytQ7nBYYWrnTotevQ3fGQBiVfvZlLIXAqTMDJNDTTZSSUOV37dOK5431AFKZz820Z3eIFA9l94HzpC4tG85N38kFbboBgU/HpQyGuNjQ9B8RYM57U3bBxayXh5IyE6DTp08gu5Utn7e5fef/vDekCMBYmob68bY0G1oEyTMtPlc+WXPupoHjh7WutpiqLtOE+134RMIDDKX2LlfGw/VGNzv5Zu4oKUpFEtt+UoqTFUmJtyg1dHOhli1XPDcGQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Right now, kernel faults on user addresses, such as those from copy_from_user() and copy_to_user(), unconditionally fall back to the mmap_lock path. This patchset switches them to the per-VMA lock path for three reasons: 1. These faults are common. On a typical Ubuntu system, hundreds of kernel faults on user addresses occur every second in python3, apt-esm-hook, package-data-do, teamviewerd, bash, scudo, cscope, gnome-shell, systemd, and many other applications. Handling these faults under mmap_lock unnecessarily increases lock contention. 2. It removes one obstacle to simplifying filemap_fault(). Matthew has proposed removing the page fault retry path and performing I/O while holding locks [1]. Based on that approach, Hongru has already reported regressions caused by performing I/O under mmap_lock for kernel faults on user addresses [2]. This patchset removes that obstacle. Another source of I/O under mmap_lock is GUP, which also relies on mmap_lock today and has been reported by Hongru to exhibit similar regressions [2]. It appears Rik van Riel may be addressing this separately [3]. 3. The current implementation is inconsistent. Although kernel faults on user addresses always fall back to mmap_lock, arch/*/mm/fault.c still performs !user_mode checks in the per-VMA lock path, making that code effectively dead. As an RFC, this patchset demonstrates the approach on x86 and arm64 only. Other architectures will need to be updated as well. [1] https://lore.kernel.org/linux-mm/20260625195040.2508362-1-willy@infradead.org/ [2] https://lore.kernel.org/linux-mm/20260712132759.2030823-1-zhanghongru@xiaomi.com/ [3] https://lore.kernel.org/linux-mm/20260724222934.1463812-1-riel@surriel.com/ Barry Song (Xiaomi) (2): x86/mm: use VMA lock for kernel faults on user addresses arm64/mm: use VMA lock for kernel faults on user addresses arch/arm64/mm/fault.c | 4 +++- arch/x86/mm/fault.c | 3 --- 2 files changed, 3 insertions(+), 4 deletions(-) -- 2.39.3 (Apple Git-146)