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Sun, 16 Nov 2025 10:11:58 -0800 (PST) Received: from [127.0.0.1] ([101.32.222.185]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8b2e089b436sm305447785a.45.2025.11.16.10.11.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 16 Nov 2025 10:11:58 -0800 (PST) From: Kairui Song Subject: [PATCH v2 00/19] mm, swap: swap table phase II: unify swapin use swap cache and cleanup flags Date: Mon, 17 Nov 2025 02:11:41 +0800 Message-Id: <20251117-swap-table-p2-v2-0-37730e6ea6d5@tencent.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-B4-Tracking: v=1; b=H4sIAN0TGmkC/13Myw6CMBCF4Vchs3ZMaeUiK9/DsChlkEm0bdoGN YR3txJXLv+TnG+FSIEpQlesEGjhyM7mkIcCzKztjZDH3CCFrEohGoxP7THp4U7oJTajEm1NlVG thvzxgSZ+7d61zz1zTC68d34pv+tPkuc/aSlRoBpPalJDTUbJSyJryKajcQ/ot237AD4WwOysA AAA X-Change-ID: 20251007-swap-table-p2-7d3086e5c38a To: linux-mm@kvack.org Cc: Andrew Morton , Baoquan He , Barry Song , Chris Li , Nhat Pham , Yosry Ahmed , David Hildenbrand , Johannes Weiner , Youngjun Park , Hugh Dickins , Baolin Wang , Ying Huang , Kemeng Shi , Lorenzo Stoakes , "Matthew Wilcox (Oracle)" , linux-kernel@vger.kernel.org, Kairui Song , linux-pm@vger.kernel.org X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=ed25519-sha256; t=1763316712; l=7592; i=kasong@tencent.com; s=kasong-sign-tencent; h=from:subject:message-id; bh=/P/YM4pb1/fXFf2POywLNCj1z51ehE/HU6r87l47Vdw=; b=EUaXCzADQCSzNT8+jGIQ3f0BDmf422WLWvvUiMFAD/rDhinkZoSugxzI7X6PaRoxbhZssy2A0 jhMadHnaNn5DO/TTGq1sMJhahSMsNPTkABUeo+AI0NZVwB4ZGEMw8KP X-Developer-Key: i=kasong@tencent.com; a=ed25519; pk=kCdoBuwrYph+KrkJnrr7Sm1pwwhGDdZKcKrqiK8Y1mI= This series removes the SWP_SYNCHRONOUS_IO swap cache bypass swapin code and special swap flag bits including SWAP_HAS_CACHE, along with many historical issues. The performance is about ~20% better for some workloads, like Redis with persistence. This also cleans up the code to prepare for later phases, some patches are from a previously posted series. Swap cache bypassing and swap synchronization in general had many issues. Some are solved as workarounds, and some are still there [1]. To resolve them in a clean way, one good solution is to always use swap cache as the synchronization layer [2]. So we have to remove the swap cache bypass swap-in path first. It wasn't very doable due to performance issues, but now combined with the swap table, removing the swap cache bypass path will instead improve the performance, there is no reason to keep it. Now we can rework the swap entry and cache synchronization following the new design. Swap cache synchronization was heavily relying on SWAP_HAS_CACHE, which is the cause of many issues. By dropping the usage of special swap map bits and related workarounds, we get a cleaner code base and prepare for merging the swap count into the swap table in the next step. Test results: Redis / Valkey bench: ===================== Testing on a ARM64 VM 1.5G memory: Server: valkey-server --maxmemory 2560M Client: redis-benchmark -r 3000000 -n 3000000 -d 1024 -c 12 -P 32 -t get no persistence with BGSAVE Before: 460475.84 RPS 311591.19 RPS After: 451943.34 RPS (-1.9%) 371379.06 RPS (+19.2%) Testing on a x86_64 VM with 4G memory (system components takes about 2G): Server: Client: redis-benchmark -r 3000000 -n 3000000 -d 1024 -c 12 -P 32 -t get no persistence with BGSAVE Before: 306044.38 RPS 102745.88 RPS After: 309645.44 RPS (+1.2%) 125313.28 RPS (+22.0%) The performance is a lot better when persistence is applied. This should apply to many other workloads that involve sharing memory and COW. A slight performance drop was observed for the ARM64 Redis test: We are still using swap_map to track the swap count, which is causing redundant cache and CPU overhead and is not very performance-friendly for some arches. This will be improved once we merge the swap map into the swap table (as already demonstrated previously [3]). vm-scabiity =========== usemem --init-time -O -y -x -n 32 1536M (16G memory, global pressure, simulated PMEM as swap), average result of 6 test run: Before: After: System time: 282.22s 283.47s Sum Throughput: 5677.35 MB/s 5688.78 MB/s Single process Throughput: 176.41 MB/s 176.23 MB/s Free latency: 518477.96 us 521488.06 us Which is almost identical. Build kernel test: ================== Test using ZRAM as SWAP, make -j48, defconfig, on a x86_64 VM with 4G RAM, under global pressure, avg of 32 test run: Before After: System time: 1379.91s 1364.22s (-0.11%) Test using ZSWAP with NVME SWAP, make -j48, defconfig, on a x86_64 VM with 4G RAM, under global pressure, avg of 32 test run: Before After: System time: 1822.52s 1803.33s (-0.11%) Which is almost identical. MySQL: ====== sysbench /usr/share/sysbench/oltp_read_only.lua --tables=16 --table-size=1000000 --threads=96 --time=600 (using ZRAM as SWAP, in a 512M memory cgroup, buffer pool set to 3G, 3 test run and 180s warm up). Before: 318162.18 qps After: 318512.01 qps (+0.01%) In conclusion, the result is looking better or identical for most cases, and it's especially better for workloads with swap count > 1 on SYNC_IO devices, about ~20% gain in above test. Next phases will start to merge swap count into swap table and reduce memory usage. One more gain here is that we now have better support for THP swapin. Previously, the THP swapin was bound with swap cache bypassing, which only works for single-mapped folios. Removing the bypassing path also enabled THP swapin for all folios. The THP swapin is still limited to SYNC_IO devices, the limitation can be removed later. This may cause more serious THP thrashing for certain workloads, but that's not an issue caused by this series, it's a common THP issue we should resolve separately. Link: https://lore.kernel.org/linux-mm/CAMgjq7D5qoFEK9Omvd5_Zqs6M+TEoG03+2i_mhuP5CQPSOPrmQ@mail.gmail.com/ [1] Link: https://lore.kernel.org/linux-mm/20240326185032.72159-1-ryncsn@gmail.com/ [2] Link: https://lore.kernel.org/linux-mm/20250514201729.48420-1-ryncsn@gmail.com/ [3] Suggested-by: Chris Li Signed-off-by: Kairui Song --- Series is almost identical to V1, mostly comment update and build fix, and rebase to resolve conflicts and for easier review. Stress test and performance test is looking good and basically same as before. Changes in v2: - Rebased on latest mm-new to resolve conflicts, also appliable to mm-unstable. - Imporve comment, and commit messages in multiple commits, many thanks to [Barry Song, YoungJun Park, Yosry Ahmed ] - Fix cluster usable check in allocator [ YoungJun Park] - Improve cover letter [ Chris Li ] - Collect Reviewed-by [ Yosry Ahmed ] - Fix a few build warning and issues from build bot. - Link to v1: https://lore.kernel.org/r/20251029-swap-table-p2-v1-0-3d43f3b6ec32@tencent.com --- Kairui Song (18): mm, swap: rename __read_swap_cache_async to swap_cache_alloc_folio mm, swap: split swap cache preparation loop into a standalone helper mm, swap: never bypass the swap cache even for SWP_SYNCHRONOUS_IO mm, swap: always try to free swap cache for SWP_SYNCHRONOUS_IO devices mm, swap: simplify the code and reduce indention mm, swap: free the swap cache after folio is mapped mm/shmem: never bypass the swap cache for SWP_SYNCHRONOUS_IO mm, swap: swap entry of a bad slot should not be considered as swapped out mm, swap: consolidate cluster reclaim and check logic mm, swap: split locked entry duplicating into a standalone helper mm, swap: use swap cache as the swap in synchronize layer mm, swap: remove workaround for unsynchronized swap map cache state mm, swap: sanitize swap entry management workflow mm, swap: add folio to swap cache directly on allocation mm, swap: check swap table directly for checking cache mm, swap: clean up and improve swap entries freeing mm, swap: drop the SWAP_HAS_CACHE flag mm, swap: remove no longer needed _swap_info_get Nhat Pham (1): mm/shmem, swap: remove SWAP_MAP_SHMEM arch/s390/mm/pgtable.c | 2 +- include/linux/swap.h | 77 ++--- kernel/power/swap.c | 10 +- mm/madvise.c | 2 +- mm/memory.c | 275 ++++++++-------- mm/rmap.c | 7 +- mm/shmem.c | 75 ++--- mm/swap.h | 70 +++- mm/swap_state.c | 340 +++++++++++++------- mm/swapfile.c | 848 +++++++++++++++++++++---------------------------- mm/userfaultfd.c | 10 +- mm/vmscan.c | 1 - mm/zswap.c | 4 +- 13 files changed, 842 insertions(+), 879 deletions(-) --- base-commit: 41218ede767f6b218185af65ce919d0cade75f6b change-id: 20251007-swap-table-p2-7d3086e5c38a Best regards, -- Kairui Song