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McKenney" Cc: Andrew Morton , Catalin Marinas , puranjay@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH] mm/kmemleak: report RCU-tasks quiescent states during the scan Message-ID: References: <20260720-kmemleak_rcu_task-v1-1-5b460ade777d@debian.org> <20260720153917.2e428e489cd0873ac6d4e6da@linux-foundation.org> <5b83b0a0-708a-458a-bdbd-41c6d4610349@paulmck-laptop> <39c90e82-f785-4b47-9571-dd4b1339d685@paulmck-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Debian-User: leitao X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 154831C0012 X-Stat-Signature: kk5rz38jx5f43nd9harm8ezsmmhgjrbz X-Rspam-User: X-HE-Tag: 1785168144-102838 X-HE-Meta: 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 O7eXUrDH +wH3LjDt8pDkAqBSM5ZUbMD19isIBPtO4zxiTNVfRXqAWwnWbyBdpLX8BK2CEX5B02Y4EEJ1KCsfZd8Y4XsrWYXxQTQfd7qXb1NMKMiW5iFEwIJpDnjoETku48cA+ykhFWaRIhkJqYqjYd4VITVUPcL0qukZE75p6RdlD5Vd3eqWTwCsca8PZ8vZ7Ls0zIKiC1aqgFLBN9bCmNuxH0++K8eXYzWLzqvlblWbaQOgXMM0XeI+VQ8YdM1Sa/Z1LCprF3olETB2Uu81sp2j8r1nZlONQ1Ttm2N+YFLMk1P0vbrilvToIX0SnACMDIw3kNGYw8t7q9TyeppG97lU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jul 27, 2026 at 06:04:37AM -0700, Breno Leitao wrote: > > > The document was not a waste of time. it helped people like me to > > > understand what the issue is, and what are the decisions we have ahead > > > of us. > > > > > > Back to what is the best decision/design, I honestly don't have an > > > opinion, but I am happy to get more data about Meta production servers > > > to help with the decision. > > > > Looking forward to seeing what you come up with! > > I found 3 different cases on Meta fleet, where rcu task stalls show up: > > 1) kmemleak -> This patch solves it > 2) KVM / kcompactd > * Holdout: kcompactd0 (pid 1216), state:R, nvcsw frozen at > 716336/716336 across three reports 10 min apart (stuck ≥20 min in > one compaction pass) > * This is coming from: migrate_pages -> > kvm_mmu_notifier_invalidate_range_start -> > tdp_mmu_next_root -> tdp_mmu_zap_leafs > > 3) Nvidia driver > * stuck in nv_procfs_read_lock_params > > Given we can't do much about 3) and 1) is now closed, I will spend some > time geting more data and possible a fix for 2. Quick update about item number 2), I've investigated and found that migrate_pages_batch() unmaps each folio before moving it, and every unmap runs the mmu_notifier invalidate callbacks. On KVM hosts try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() -> tdp_mmu_zap_leafs(), which is expensive. Patch at: https://lore.kernel.org/all/20260727-kcompact-v1-1-bdfefddd6874@debian.org/