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[85.239.239.237]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2dd2ceb7720sm57859485ad.37.2026.09.14.19.55.30 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Mon, 14 Sep 2026 19:55:36 -0700 (PDT) From: Lian Wang To: Ravi Jonnalagadda Cc: "Lian Wang (ProcessMission)" , sj@kernel.org, akinobu.mita@gmail.com, damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, akpm@linux-foundation.org, corbet@lwn.net, bijan311@gmail.com, ajayjoshi@micron.com, honggyu.kim@sk.com, yunjeong.mun@sk.com, rientjes@google.com, weixugc@google.com, jic23@kernel.org, gourry@gourry.net, Kunwu Chan Subject: Re: [RFC PATCH v2 0/9] mm/damon: hardware-sampled access reports Date: Tue, 15 Sep 2026 10:54:51 +0800 Message-ID: <20260915025514.2434-1-lianux.mm@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260910171623.6638-1-ravis.opensrc@gmail.com> References: <20260910171623.6638-1-ravis.opensrc@gmail.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: "Lian Wang (ProcessMission)" Hi Ravi and SJ, Thank you for sharing this work. The report path looks potentially useful for a DAMON observation-granularity problem that we are investigating. The scenario originated from an issue reported by SXF, and we are planning to share and discuss it at LPC. We have now reproduced the underlying observation difference on both a local PC and an x86 server. The server run below is our larger, controlled reproduction. Our test monitors a QEMU process whose guest memory is backed by a shared tmpfs file. The guest repeatedly writes one 4 KiB page in every 2 MiB range of a resident 48 GiB data set. The unique 4 KiB spatial coverage of one sweep is about 96 MiB, or 0.195% of that data set. Using vaddr DAMON with a 500 ms sampling interval, 20 second aggregation interval, and 500--2000 regions, two opposite-order runs reported a mean hot ratio of 1.751% without shmem PMD mappings and 75.836% with them. A control that writes the full 2 MiB ranges reported 75.366% with PMD mappings. Hence, in this workload the current observation does not distinguish spatially sparse 4 KiB activity from dense activity once the large mappings are present. We are testing existing DAMON strategy mitigations first. As a next step, we plan to replay recorded access addresses through the report/probe decision path, independently of a live PMU, and check whether DAMOS can distinguish the sparse and dense cases. In that experiment, raw report frequency and unique 4 KiB spatial coverage will remain separate quantities; lack of reports will remain unknown when source coverage or loss is insufficient. If the report evidence makes the decision reliable, the response we want to evaluate is an internal, bounded PMD mapping demotion (splitting the mapping while retaining the underlying THP folio), rather than exposing physical folio split as a DAMOS action. We will first keep the response stat-only and archive the would-act ranges. After the replay result is repeatable, we can replace the replay producer with an appropriate hardware source and share the results. This seems like a useful consumer experiment for the report interface, even if the vaddr and optimized transport parts belong to the later phase. Please let us know if this direction conflicts with the intended report semantics. We will share the strategy, replay, and mapping-decision results once they are repeatable. We are also reviewing and testing this series, and will follow up with the appropriate tag(s) for the exact revision if that work justifies them. Thanks, Lian On Thu, 10 Sep 2026 10:16:14 -0700 Ravi Jonnalagadda wrote: > This series lets DAMON take its access information from a hardware sampler > instead of from a page-table scan, and lets a scheme's score be weighted by what > that sampler reported. Sent using hkml (https://github.com/sjp38/hackermail)