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Mon, 02 Nov 2026 13:24:47 +0000 X-Mizu-Trace-ID: 19717573b0124653 X-Migadu-Flow: FLOW_OUT From: xu.xin@linux.dev To: akpm@linux-foundation.org, david@kernel.org Cc: agordeev@linux.ibm.com, gerald.schaefer@linux.ibm.com, hca@linux.ibm.com, gor@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, mhiramat@kernel.org, oleg@redhat.com, peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, james.clark@linaro.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, kas@kernel.org, xu.xin@linux.dev, chengming.zhou@linux.dev, matthew.brost@intel.com, joshua.hahnjy@gmail.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, riel@surriel.com, harry@kernel.org, jannh@google.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v2 0/4] mm/ksm: use per-VMA locking to reduce mmap_lock contention Date: Mon, 2 Nov 2026 21:24:16 +0800 Message-Id: <20261102132416.300084-1-xu.xin@linux.dev> X-Mailer: git-send-email 2.34.1 X-Archived-By: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 82D9480008 X-Stat-Signature: tyc6wtox86f67rpy7dopnjdko4ojfrx8 X-Rspamd-Server: rspam01 X-Spam-Status: No, score=-13.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1793625890-283551 X-HE-Meta: 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 Wo9bIP5y uHB5Hf1CIJI/uVj2W6r/rvrpYv+k2Wljiosi65ATdB8RK9i5YZ9MgaysweqpjvlzOCTJkwGMBdXUTIFNGOWMvUTWTnHW3QZaaFtLR2XrxLWa9XDmZM2BDCVVKMHyWvoc5mSpAqbTCNI37/Za/XgB52i0W3j1qrJv6fs2Fvw2kp2qW3wdy4IWA08nAbJdgN84XOISRzfPJp4hLfFR8i/gGPUuAV7c+wzz3lam0Wus9XQqHq7U6HSl1XGUi7HC8psRUWXJhs4PyTCGJ2hI46OJZK9KCdum80BBRynlyRe78E9v4ICYR8H0gjUx58zoWLePTjnb8187zdWy3ey5U5j0cPicOgdId5ctkiwbo Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 From: Xu Xin KSM scans VM_MERGEABLE VMAs and currently protects each scan with mmap_read_lock(). Per-VMA locking allows KSM to read-lock only the VMA it is actually interested in, so that unrelated mmap()/munmap() activity in the same mm no longer blocks ksmd. This series is organized as follows: Patch 1 removes an unused 'vma' member from struct folio_walk. It has never been used since its introduction and is pure cleanup. Patch 2 adds a 'walk_lock' member to struct folio_walk and extends folio_walk_start() to assert the required locking mode. Existing callers are converted to pass PGWALK_RDLOCK, so there is no functional change. This prepares folio_walk_start() for callers that hold a per-VMA read lock instead of mmap_read_lock(), which is needed by the Patch 4. No functional change. Patch 3 replaces the boolean 'lock_vma' argument of break_ksm() with a KSM-local enum ksm_walk_lock and builds the mm_walk_ops on the stack, so the function-pointer table stays const and there is no shared mutable state. No functional change. Patch 4 introduces find_mergeable_vma_locked(), which uses the universal per-VMA locking helper vma_start_read_unlocked() to look up and read-lock a VM_MERGEABLE VMA without taking mmap_read_lock(). All KSM call sites that previously used find_mergeable_vma() under mmap_read_lock() are converted to the new helper, and the locking in get_mergeable_page() is switched to PGWALK_VMA_RDLOCK_VERIFY so that folio_walk_start() can verify the per-VMA lock is held. Because ksmd no longer holds mmap_read_lock() while walking, __ksm_exit() must also write-lock every VMA (in addition to mmap_write_lock()) to keep serializing against an in-flight ksmd walk before the page tables are freed. A microbenchmark was run to measure the time KSM takes to merge a victim region under mmap_lock contention. Under interference from 4 churner threads, the merge time of the per-VMA KSM-optimized kernel is significantly reduced by 50%. Changelog: v2: - Patch 1: rewrote the commit message to explain the introducing commit, how the VMA is actually used, and the audit performed; dropped the Fixes: tag. - Patch 3: * replaced the boolean 'lock_vma' with a KSM-local enum ksm_walk_lock instead of exposing the internal enum page_walk_lock as a parameter; * build the mm_walk_ops on the stack rather than mutating a global struct, so the function-pointer table stays const and there is no shared mutable state; rewrote the commit message. - Patch 4: * Fixed a per-VMA read-lock leak in find_mergeable_vma_locked() when the VMA is not mergeable; made __ksm_exit() write-lock every VMA (vma_start_write()) in addition to mmap_write_lock(), so that it still serializes against a ksmd walk that holds only a per-VMA read lock. Xu Xin (4): mm/pagewalk: delete the unused member mm: make folio_walk_start()'s locking asserts scalable mm/ksm: make break_ksm() more scalable ksm: add find_mergeable_vma_locked() to use per-VMA locking arch/s390/mm/fault.c | 4 +- include/linux/pagewalk.h | 2 +- kernel/events/uprobes.c | 4 +- mm/huge_memory.c | 4 +- mm/ksm.c | 124 +++++++++++++++++++++++++++------------ mm/migrate.c | 8 ++- mm/pagewalk.c | 10 +++- mm/rmap.c | 4 +- 8 files changed, 116 insertions(+), 44 deletions(-) -- 2.34.1