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Tue, 08 Sep 2026 07:46:17 +0000 X-Mizu-Trace-ID: 539c04568d93ce89 X-Migadu-Flow: FLOW_OUT Message-ID: <1fc41803-a0b3-48ca-84a6-caf965fb11f9@linux.dev> Date: Tue, 8 Sep 2026 15:46:08 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 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: <20260908030335.96549-1-songmuchun@bytedance.com> <20260908030335.96549-7-songmuchun@bytedance.com> From: Qi Zheng In-Reply-To: <20260908030335.96549-7-songmuchun@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: B64CA140005 X-Stat-Signature: brmhurgzzapfimrf9wu73uzkffdank7i X-HE-Tag: 1788853588-354202 X-HE-Meta: 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 FQffgSxO 6zbAj4vLZUneafKGuKwdQQwuZWbMMOJ5SV22EekocN+ZL/qoM/xn7vwRPm5DOY1Y95dnywyuOvNUNseUOlCuwDWjIwnc2zogD7Xx9lW69S/HeNAexq9XHqRySh5/qOqjfMDrh4vG/wRfMZY1wFTsZvDhSqe5aGPAj2wUDL6I7GfRNo27kbc0+Ck0hvXohxXe5QPagG6DRQHQ+1shJ222027d8mZNV67kg6frRT1D2aA18S35D5MoJ6RIKE7u5izc2jOCr/TSV60cH3upvcYn2upWUhhhjJUwoxaqso9nvTtgJRMeIUtaBmYqkpNWTIDoYJ/ZDycYVNRaqK2c= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/8/26 11:03 AM, 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. > > The optimization is enabled only for DEV-DAX through pgmap->vmemmap_shift, > which is assigned when a DEV-DAX device is initialized. Unlike FS-DAX, > DEV-DAX does not modify tail struct pages, so sharing them is safe. Got it — adding this description makes things significantly clearer. > > Since the shared tail page can now back ZONE_DEVICE vmemmap mappings, > initialize its entries with PG_reserved for device zones. Also skip > poisoning vmemmap-optimizable sections while their struct pages may be > shared. > > Signed-off-by: Muchun Song > --- > v2: > - Explain why sharing tail vmemmap pages is safe for DEV-DAX > (suggested by Qi Zheng) > --- > include/linux/mmzone.h | 10 +++++++++ > mm/memory_hotplug.c | 5 +++-- > mm/sparse-vmemmap.c | 47 ++++++++++++++---------------------------- > 3 files changed, 28 insertions(+), 34 deletions(-) > Acked-by: Qi Zheng Thanks, Qi