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Tue, 08 Sep 2026 09:12:22 +0000 X-Mizu-Trace-ID: e6379ba285176541 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 v2 09/11] mm/sparse-vmemmap: drop the extra tail page from device DAX reservation From: Muchun Song In-Reply-To: Date: Tue, 8 Sep 2026 17:12:07 +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: References: <20260908030335.96549-1-songmuchun@bytedance.com> <20260908030335.96549-10-songmuchun@bytedance.com> To: Qi Zheng X-Mailer: Apple Mail (2.3864.700.51.1.1) X-Stat-Signature: ypup674zmm8fzbbee36zpuumk78ttjp6 X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 9BDA040007 X-Rspam-User: X-HE-Tag: 1788858744-249804 X-HE-Meta: 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 qhm6yYW4 zq+Eq/6wPyTfgAn4esfYhE8FaP6keomFft8AOVupyqYtlpao4Tw13myVa6cnMMFObaH8Y4pEC2+GCDnMPcWC8T9toJZi84NRntlRtk3BxBCdnH3XOYPiGeBynRXEKqDWFXRps+bCzyt2A38rzQj7C0sdO4BJpjVPfwVjgImRCHk3NDUAD7fVUlAgT/cWkdtVTMtWVbek7G4m8wAvvaeEQc+UXr33SOAhgHsRo9077UipK+IxQAltlbJWFf1cEJmtGmhTOuhWQyfQsKqRZgRpbP57LCyX1vO2SbajpmeY8lNMV0ghLUl+iObe/1HB6Ml6FnMJ5atJhYUKxBxWMautEz/bKvsOp4AE+aA/x Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Sep 8, 2026, at 17:09, Qi Zheng wrote: >=20 >=20 >=20 > On 9/8/26 11:03 AM, Muchun Song wrote: >> The device DAX vmemmap population still reserves one extra tail = vmemmap >> page after the head page. >> Drop that extra reservation and let the shared tail page cover all = tail >> vmemmap pages after the head page, so DAX follows the same = reservation >> model as HugeTLB. >> This reduces the reserved vmemmap pages for optimized DAX mappings to >> one and removes the now-unneeded first-tail population from the = generic >> and powerpc paths to simplify the code as well. >> Signed-off-by: Muchun Song >> --- >> arch/powerpc/mm/book3s64/radix_pgtable.c | 46 = ++---------------------- >> include/linux/mm.h | 3 +- >> mm/mm_init.c | 2 +- >> mm/sparse-vmemmap.c | 13 ++----- >> 4 files changed, 7 insertions(+), 57 deletions(-) >=20 > Looks like sashiko's response [1] is a false positive. >=20 > [1]. = https://sashiko.dev/#/patchset/20260908030335.96549-1-songmuchun%40bytedan= ce.com >=20 > __SetPageReserved() in vmemmap_shared_tail_page() marks the struct = page > entries stored inside the shared vmemmap backing page, i.e. the = metadata > templates that later represent DEV-DAX tail pages. It does not mark = the > struct page of the backing page itself. >=20 > On hot-remove, free_vmemmap_pages() is called with pte_page(*pte), = which > is the backing page. That page is not reserved, so the teardown goes > through __free_pages(), not free_reserved_pages(). >=20 > Right? Absolutely right. >=20 > If so: >=20 > Acked-by: Qi Zheng Thanks.