[Bug 2170644] Re: arm64: munmap performance regression

Submission to mail list here: https://lists.ubuntu.com/archives/kernel-team/2026-October/172906.html -- You received this bug notification because you are subscribed to linux in Ubuntu. Matching subscriptions: Bgg, Bmail, Nb https://bugs.launchpad.net/bugs/2170644 Title: arm64: munmap performance regression Status in linux package in Ubuntu: New Bug description: [Impact] Starting with Noble linux-6.8.0-146.146, users on arm64 architectures began reporting 2-3x performance regressions on workloads that performed munmap(2) calls in their critical path. One affected workload was git, which with a large repo containing lots of packfiles, experienced a 3x increase in system time for fetches post-upgrade. This doesn't affect all arm64 implementations equally. When testing across the Graviton fleet, Neoverse-N1 (6g) and Neoverse-V1 (7g) observed the impact as a 220-300% regression, whereas Neoverse-V2 (8g) and Neoverse-V3 (9g) saw something closer to 5%. I haven't had an opportunity to test Azure or NVIDIA systems for a wider comparison. Running a build without CONFIG_ARM64_ERRATUM_4118414 restored the missing performance. Upon inspection, Ubuntu seems not to have backported the fix from this patch: a8f78680ee6b ("arm64: tlb: Optimize ARM64_WORKAROUND_REPEAT_TLBI") Apply it with CONFIG_ARM64_ERRATUM_4118414 enabled also restored the missing performance. This fix was part of LTS 6.6.143 and LTS 6.12.94, which were purportedly included in 6.8.0-146.146. I tested vanilla builds of 6.12.94 on the same hardware and further confirmed no regression was present. This patch appears to have been lost in the shuffle with both the LTS merge for this release and the inclusion of the fixes for CVE-2025-10263. I additionally scanned the 6.6.143 and 6.12.94 patches for other absent backports and identified the following other candidates as not present: 3c2d42b8ee34 ("mm/memory-failure: fix hugetlb_lock AA deadlock in get_huge_page_for_hwpoison") 8d878059924f ("mm/huge_memory: update file PMD counter before folio_put()") ba23f58de896 ("mm/migrate: don't call folio_putback_active_hugetlb() on dst hugetlb folio") 2c85c61d1332 ("HID: pass the buffer size to hid_report_raw_event") 165573e41f2f ("tcp: secure_seq: add back ports to TS offset") fba0510cd626 ("pmdomain: imx: fix OF node refcount") da3039e91d1f ("ALSA: timer: Forcibly close timer instances at closing") 8473c3a197b5 ("pinctrl: mcp23s08: Initialize mcp->dev and mcp->addr before regmap init") a7767290e77c ("net/mlx5: Use effective affinity mask for IRQ selection") 2b5ff4db5d7a ("ALSA: seq: dummy: fix UMP event stack overread") 1aee05e814d2 ("erofs: fix use-after-free on sbi->sync_decompress") I don't have context on whether these omissions are intentional. The tcp secure_seq patch, for example, makes kabi changes. I opted not to include the remainders in this list in order to expedite the resolution of the munmap regression. I can follow-up with a subsequent series for these other patches, if necessary. [Fix] Cherry pick the following fix from stable-6.6.y: a8f78680ee6b ("arm64: tlb: Optimize ARM64_WORKAROUND_REPEAT_TLBI") [Test] Constructed a munmap microbenchmark that unmapped a 7GiB worth of 1MiB file-backed segments. With ARM64_ERRATUM_4118414 enabled but optimizations missing observed 200-300% increase on some arm64 platforms. Applied the fix listed above and re-ran the tests and validated that baseline performance from before ARM64_ERRATUM_4118414 was enabled returned. [Where problems can occur] This fix has been in Linux since 7.0. There have been no followup fixes identifying this patch as problematic with a "Fixes" tag. The likelihood of problems is low, but if they were to occur it would involve tlb flushing edge cases which do present as subtle mm errors and can be quite fiddly to track down. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2170644/+subscriptions