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Sat, 05 Sep 2026 09:08:31 +0000 X-Mizu-Trace-ID: 6189846893d99aae 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 06/11] mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages From: Muchun Song In-Reply-To: Date: Sat, 5 Sep 2026 17:08:12 +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: <861645FA-43D5-4619-82B6-7A0C889F95B8@linux.dev> References: <20260831075342.57563-1-songmuchun@bytedance.com> <20260831075342.57563-7-songmuchun@bytedance.com> To: Qi Zheng X-Mailer: Apple Mail (2.3864.700.51.1.1) X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 489361C0003 X-Stat-Signature: n317ofm5jzbxdwdiwurc3xagyp1hyobc X-HE-Tag: 1788599313-649142 X-HE-Meta: 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 Iu8l69z6 /SPsepZs8QoyTyaf+wnGEk4lLSHUO9WreQSqOJ9j06Hzq6TvhrlxjkNeXBzE7Vr+WjOv2+cMEoBMvvR9iajT9YVuutYT7eylA7W45hV8QRhcXL/uKfTOCpqotlErja7bFiM3YrLgHPzqmQBNOJFFmL8rLV6uEPWx3MsWtz8ZWYkXKZx6crT84QMOYoMCVT98odMZ7q11Y1BY0tEtW+b0rm/WkyScMgCtNfKeZ22vXTDZQWJhqEXoxPoQtb87MeslYc36sba7hX+NNgYW+rWqxy0oCaQOpfTiGVO3lsEO4lw2+8zTa2e9hxW4nfcyFlbJYaXCX6YyY4wDqw5+WK0SRVFhpAbyuAiaqd6cX 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 16:35, Qi Zheng wrote: >=20 >=20 >=20 > 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, >=20 > Could you explain this in more detail? >=20 > As sashiko pointed out [1], it looks like this assumption is > problematic: dax_folio_reset_order() may end up modifying all shared > tail pages? I believe Sashiko's report is a false positive, because the issue = Sashiko raised is based on assumptions from code analysis for FS-DAX. = Unfortunately, the current implementation that supports HVO is DEV-DAX, and DEV-DAX = does not modify tail struct pages. Moreover, this patch itself is intended to migrate DEV-DAX to section-based vmemmap optimization, so there is no = issue at all. BTW, the current logic for determining whether DEV-DAX applies HVO is = based on pgmap->vmemmap_shift, and pgmap->vmemmap_shift is only assigned = during the initialization of DEV-DAX devices. Muchun, Thanks. >=20 > [1]. = https://sashiko.dev/#/patchset/20260831075342.57563-1-songmuchun%40bytedan= ce.com >=20 >> initialize its entries with PG_reserved for device zones. Also skip >> poisoning vmemmap-optimizable sections while their struct pages may = be >> shared.