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Sat, 05 Sep 2026 08:35:42 +0000 X-Mizu-Trace-ID: 41100b57e1186b92 X-Migadu-Flow: FLOW_OUT Message-ID: Date: Sat, 5 Sep 2026 16:35:31 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 06/11] mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages To: Muchun Song , Andrew Morton , David Hildenbrand , Oscar Salvador , Madhavan Srinivasan , Michael Ellerman , Jonathan Corbet Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-doc@vger.kernel.org, Muchun Song , Lorenzo Stoakes , Mike Rapoport , Nicholas Piggin , Christophe Leroy , Randy Dunlap References: <20260831075342.57563-1-songmuchun@bytedance.com> <20260831075342.57563-7-songmuchun@bytedance.com> From: Qi Zheng In-Reply-To: <20260831075342.57563-7-songmuchun@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 4237D1A0008 X-Stat-Signature: z3fr41g5wdmdu1rjha5ea58nc65z9ex1 X-HE-Tag: 1788597343-818200 X-HE-Meta: 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 H7tJYz7K b1qY215Mfrur7H0EYz6YxDj8aDVGfWhyjSsjE6GlF9e6c63M02tZJ8WEoiSVQHpLmcmD3BiAX1Vbz5pYFw4cs3boy+qvzaJ1BWZ6XqnbPmhjlyCTQ/5KxFxWLiTvqk02oSmBarY07GtWauBh5PjWwdyZbo7deoHx4c8F3UcTKoYmFgSY0W36B0sF7Tam3GCDA49VuEQig+hcIxuFwbYVe9Snv6yfCPNZvrQnFy7lodlZIwqRf0aG1dEorvWldQL3hx21dMqEukfHB/i2XL2rWw8ZoRHdipn0JVMG3ccIdYspL2VgM8YvR1KTiJXhpH6HabDOCogGKDn2tGO7n+jr+eb9Knyy6KI6joG7jZkWnAjOkMx8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/31/26 3:53 PM, Muchun Song wrote: > HugeTLB vmemmap optimization now uses per-zone shared tail vmemmap pages. > Device DAX has not been switched to that mechanism yet. > > Switch device DAX to vmemmap_shared_tail_page() as well. This aligns DAX > with HugeTLB by using the common per-zone shared tail vmemmap page. > > Since the shared tail page can now back ZONE_DEVICE vmemmap mappings, Could you explain this in more detail? As sashiko pointed out [1], it looks like this assumption is problematic: dax_folio_reset_order() may end up modifying all shared tail pages? [1]. https://sashiko.dev/#/patchset/20260831075342.57563-1-songmuchun%40bytedance.com > initialize its entries with PG_reserved for device zones. Also skip > poisoning vmemmap-optimizable sections while their struct pages may be > shared. >