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AJvYcCUwVDbvrwxZsvrD8dix2O/GzOAdZWFAgAjcaYIj0nBwpJ0/rRI0auq+y4N6aA/dFhfy3Igsn2TdnqMxAduLZuYEA+uR4RQx6SYH13ms X-Gm-Message-State: AOJu0YzDcRlpm4CtpGp0VTjM282dQ4ecvgqrUqYVs49jRRnTvbJaFNk9 jKBZ1HLEfvjvLlJmlcRLocCht4aRDVgaEEbI6wHV9oso8hfyhDVWOInO9SIqnA7kuGE59wO7jo8 KwL7EL1IyF5+eCyElAzA3vg== X-Received: from lg.mtv.corp.google.com ([2620:15c:211:202:fd80:ef0f:6359:fc4c]) (user=lokeshgidra job=sendgmr) by 2002:a81:a107:0:b0:604:228:7e with SMTP id y7-20020a81a107000000b006040228007emr2397004ywg.8.1707783583386; Mon, 12 Feb 2024 16:19:43 -0800 (PST) Date: Mon, 12 Feb 2024 16:19:17 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 X-Mailer: git-send-email 2.43.0.687.g38aa6559b0-goog Message-ID: <20240213001920.3551772-1-lokeshgidra@google.com> Subject: [PATCH v5 0/3] per-vma locks in userfaultfd From: Lokesh Gidra To: akpm@linux-foundation.org Cc: lokeshgidra@google.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, selinux@vger.kernel.org, surenb@google.com, kernel-team@android.com, aarcange@redhat.com, peterx@redhat.com, david@redhat.com, axelrasmussen@google.com, bgeffon@google.com, willy@infradead.org, jannh@google.com, kaleshsingh@google.com, ngeoffray@google.com, timmurray@google.com, rppt@kernel.org, Liam.Howlett@oracle.com Content-Type: text/plain; charset="UTF-8" Performing userfaultfd operations (like copy/move etc.) in critical section of mmap_lock (read-mode) causes significant contention on the lock when operations requiring the lock in write-mode are taking place concurrently. We can use per-vma locks instead to significantly reduce the contention issue. Android runtime's Garbage Collector uses userfaultfd for concurrent compaction. mmap-lock contention during compaction potentially causes jittery experience for the user. During one such reproducible scenario, we observed the following improvements with this patch-set: - Wall clock time of compaction phase came down from ~3s to <500ms - Uninterruptible sleep time (across all threads in the process) was ~10ms (none in mmap_lock) during compaction, instead of >20s Changes since v4 [4]: - Fix possible deadlock in find_and_lock_vmas() which may arise if lock_vma() is used for both src and dst vmas. - Ensure we lock vma only once if src and dst vmas are same. - Fix error handling in move_pages() after successfully locking vmas. - Introduce helper function for finding dst vma and preparing its anon_vma when done in mmap_lock critical section, per Liam Howlett. - Introduce helper function for finding dst and src vmas when done in mmap_lock critical section. Changes since v3 [3]: - Rename function names to clearly reflect which lock is being taken, per Liam Howlett. - Have separate functions and abstractions in mm/userfaultfd.c to avoid confusion around which lock is being acquired/released, per Liam Howlett. - Prepare anon_vma for all private vmas, anonymous or file-backed, per Jann Horn. Changes since v2 [2]: - Implement and use lock_vma() which uses mmap_lock critical section to lock the VMA using per-vma lock if lock_vma_under_rcu() fails, per Liam R. Howlett. This helps simplify the code and also avoids performing the entire userfaultfd operation under mmap_lock. Changes since v1 [1]: - rebase patches on 'mm-unstable' branch [1] https://lore.kernel.org/all/20240126182647.2748949-1-lokeshgidra@google.com/ [2] https://lore.kernel.org/all/20240129193512.123145-1-lokeshgidra@google.com/ [3] https://lore.kernel.org/all/20240206010919.1109005-1-lokeshgidra@google.com/ [4] https://lore.kernel.org/all/20240208212204.2043140-1-lokeshgidra@google.com/ Lokesh Gidra (3): userfaultfd: move userfaultfd_ctx struct to header file userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx userfaultfd: use per-vma locks in userfaultfd operations fs/userfaultfd.c | 86 ++----- include/linux/userfaultfd_k.h | 75 ++++-- mm/userfaultfd.c | 450 ++++++++++++++++++++++++++-------- 3 files changed, 421 insertions(+), 190 deletions(-) -- 2.43.0.687.g38aa6559b0-goog