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Sat, 05 Sep 2026 09:00:27 +0000 X-Mizu-Trace-ID: 27e0770342031b0e X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.700.51.1.1\)) Subject: Re: [PATCH 05/11] mm/sparse-vmemmap: set section order for device DAX From: Muchun Song In-Reply-To: <1143901c-5036-4b79-8135-21ce57075136@linux.dev> Date: Sat, 5 Sep 2026 17:00:08 +0800 Cc: Muchun Song , Andrew Morton , David Hildenbrand , Oscar Salvador , Madhavan Srinivasan , Michael Ellerman , Jonathan Corbet , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-doc@vger.kernel.org, Lorenzo Stoakes , Mike Rapoport , Nicholas Piggin , Christophe Leroy , Randy Dunlap Content-Transfer-Encoding: quoted-printable Message-Id: <6AEC3FC2-2365-4EB5-A022-8AB0EC111073@linux.dev> References: <20260831075342.57563-1-songmuchun@bytedance.com> <20260831075342.57563-6-songmuchun@bytedance.com> <1143901c-5036-4b79-8135-21ce57075136@linux.dev> To: Qi Zheng X-Mailer: Apple Mail (2.3864.700.51.1.1) X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 2FFED20002 X-Stat-Signature: nw96rs5hsythy1ykm94cpeb4xq7bw47o X-Rspam-User: X-HE-Tag: 1788598832-606396 X-HE-Meta: U2FsdGVkX1/eLy0Bz17MmcozTFmPmMxYf8NsWdMu0UchCgR11f++PApVulb0pgqbf9QlGh2VS1dIn2r4g9eU/48QLWnW7crNHuynX1wnx4z3+OjXd4uBY7TRUAvehptAtT5wtpe7xuLrVWYKlmOTRAPfvzpd7q92YFVmM6fVh6tXspXgztkJaTzdlLI/OyPYXbrnbYh9nxIMh8fYmElMKNzJhjG6I4faMMG/SoCdCkieriqt9180dRBVkdbFnYQ6tn6KvUqZhgWz34KGORCDdmLvfBik8ky0qn72Vnk7LIcFT8dnaNccoJxEMnyhFXPzm47aR9g0Eovq5u9yp/n8TnKlLz/9ltoc332/X51F9aMiNJ4ckkEU/BH6kU5WzhSFrVbOggGE/UOgWk6gnHp2FyjI0+262B8WS6mpDh/rbjDDL99rTFZwDoJZo3QwrtH5jy5vi+Izha8lvB9tDhS4Lh1VZuvO5CeuTiIwS5kPmjoO0YQHSTNJLIWf9GXXci2V9y3Yynkh2leSYMiu12vINCqZ5vd9f011bEGJXUpxqJP3ZW6GfluB9gwxQu+ktc5Tchw3xAAF8z9WazvfJHrBJiZpzxovtYnpBwbE7nsBi+yN2+gatdTFm99uhhRtc2bcq7gaO4ZgRFi/NQbf7SpcYTT/2ECjl00I44TSlpmOV9hk3whD+SXG/Q1+dqVOupR1LC5+6mpyS2ZZVvGz/0TGDhhaYTg7bMWAYayvkB/AC0vQFFhoTTftSoebg6bWoR05OMIiVh4+TDVY4X4M5JwtDgpgrBUSXWbxVu6uiEr5gT1H11xIuPzKqvQ+8SS64b9dTKzOtsHo8v0jMgEIFUGFOtnsQBeFl6wgT1/VAxr2Xj7UNyUnCf3sIneAYiJW2TOZpN71oGd4aWH3sBF0dfv9AqLAiL70XT9IQszTi7+kqZG8EmWNtcOSNT2JEMff/6Llbtq0VBlLy06ln2KEqfM 1jj3ZN9E 2dds7sIwgerMgl6wAeXjnyFYXllTj1JGIB0RZdCPi0K2XbLUj4X4MCtoyg2pD+ItPSDxzzFF/EW9NlOPbCQ7MgbCFrOZVfvpcSm9i4gmEI+ox9Ifev7HPXeuyM8SJzEoUPKwjn1VfqnW/OHWeDk7oz4PRIMb7xJk/D1+apP884r8HW/MlLb1AbLiPhKUdqqgoFXtbQlp+INg/CnwkoAgU/dh9qSGgmLRI8t6Hdx7LQ5fKt2A4ai/OJjOrrR6+LjF7OiL2kvpgFyuhKP1J/31X4ExK2DkS43tYZS+r Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Sep 5, 2026, at 16:20, Qi Zheng wrote: >=20 > Hi Muchun, Hi, >=20 > On 8/31/26 3:53 PM, Muchun Song wrote: >> Device DAX can use vmemmap optimization only when a full section is >> populated with a compound-page geometry. Record that geometry in the >> section order before populating the section, so later vmemmap = accounting >> and population decisions can use the section state directly. >> Clear the section order when the section becomes empty again. Also = reject >> partial additions to a section that already has optimized vmemmap = mappings, >> because a section cannot safely mix optimized and ordinary vmemmap = layouts. >=20 > Perhaps we should explain why they cannot be safely mixed. Yes, because compound_nr_pages() determines the number of struct pages = to initialize with section as the smallest granularity, they cannot be = mixed. >=20 >> Partial additions continue to use ordinary vmemmap population, so = they do >> not save vmemmap memory. Such additions are uncommon, and the lost = saving >> is negligible. >> Signed-off-by: Muchun Song >> --- >> mm/mm_init.c | 13 ++++--------- >> mm/sparse-vmemmap.c | 16 ++++++++++++---- >> 2 files changed, 16 insertions(+), 13 deletions(-) >=20 > Also, the issue sashiko reported looks like it was pre-existing, not > introduced by this patch. So: >=20 > Acked-by: Qi Zheng Thanks. >=20 > Thanks, > Qi >=20