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Thu, 24 Sep 2026 00:58:12 -0700 (PDT) From: Muchun Song To: Madhavan Srinivasan , Mike Rapoport , Andrew Morton , David Hildenbrand Cc: Michael Ellerman , Nicholas Piggin , Christophe Leroy , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Qi Zheng , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Muchun Song , muchun.song@linux.dev Subject: [PATCH v2 2/6] mm/sparse-vmemmap: support device DAX in common vmemmap path Date: Thu, 24 Sep 2026 15:52:55 +0800 Message-ID: <20260924075259.2013039-3-songmuchun@bytedance.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260924075259.2013039-1-songmuchun@bytedance.com> References: <20260924075259.2013039-1-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: 9qweicciuhx5iwxn1f8sb9bsxdifj4mo X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: A726B40002 X-HE-Tag: 1790236694-258519 X-HE-Meta: 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 WVgNl4HW DCM4oGoFjOCnf6XUqbfH0VtYJvyBoB7BGfxaBkSiMYfDsLx97xW2dc1otk0DZHXKC1iRyUITCSMxV/ZQxbWJOSGR1NrsYc2it4CtuhP9CVnsCMIV6HnQW0mCHVV/B/+NidjsHBbVHruXL0QM90rdy7MBdPSyXBCc4o5nJl8NCPhksdlNCJmgMkeISYFn7EQeBJN2Axz5PS1MUdvNHQCJIPfKEr52I6HDdpxwlLgDsDpAliBeKE33/poinoNz1xvqO7UkPV8lnrGUnH7hYdfraRS9amlmXA0xxoZrpe4MMzwGlZPuR0hHt6QGDOw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: The common vmemmap population path cannot yet handle optimized Device DAX mappings on its own. It uses pfn_to_zone() to find the shared tail page, but Device DAX populates its vmemmap at runtime before the ZONE_DEVICE span is initialized. Teach the common path to use device_zone() for runtime optimized vmemmap population while retaining pfn_to_zone() for early boot. This allows the same path to support both early boot mappings and Device DAX. The backing PFN supplied by the Device DAX-specific population path is no longer used, allowing the redundant lookup and population code to be removed later. Signed-off-by: Muchun Song --- v2: - Expand comments around slab initialization to explain zone lookup and page refcounting (suggested by Qi Zheng) --- mm/sparse-vmemmap.c | 56 +++++++++++++++++++++++++-------------------- 1 file changed, 31 insertions(+), 25 deletions(-) diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c index 919c94a36346..cdcc93714a9a 100644 --- a/mm/sparse-vmemmap.c +++ b/mm/sparse-vmemmap.c @@ -208,18 +208,39 @@ static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node, struct page *page; const unsigned int order = pfn_to_section_compound_order(pfn); - /* - * Device DAX still relies on vmemmap_populate_compound_pages() for - * head/first-tail allocation and tail-page reuse. - */ if (!vmemmap_optimizable_pfn(pfn)) return vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap); - zone = pfn_to_zone(pfn, node); + /* + * Before slab is available, vmemmap optimization is used for early + * system RAM, whose zone can be determined from the PFN. + * + * Once slab is available, only ZONE_DEVICE memory reaches this + * optimized population path. Its zone span has not been initialized + * while its vmemmap is being populated, so pfn_to_zone() cannot be + * used. Obtain ZONE_DEVICE directly from the node instead. + */ + zone = slab_is_available() ? device_zone(node) : pfn_to_zone(pfn, node); page = vmemmap_shared_tail_page(order, zone); if (!page) return NULL; + /* + * During early vmemmap population, the shared tail vmemmap backing + * page is allocated from memblock before its struct page can safely + * participate in page refcounting. Therefore, no reference can be + * held for each shared PTE mapping, and the mappings must be unshared + * before the vmemmap is depopulated. + * + * Once slab is available, the shared backing page is allocated from + * the buddy allocator and can be refcounted. Hold one reference for + * each shared PTE mapping. The architecture vmemmap teardown drops + * the reference through __free_pages() when removing the mapping, + * preventing the backing page from being freed while it is shared. + */ + if (slab_is_available()) + get_page(page); + return page_address(page); } @@ -231,27 +252,12 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in if (pte_none(ptep_get(pte))) { pte_t entry; + void *p = vmemmap_alloc_pte(pfn, node, altmap); - if (ptpfn == (unsigned long)-1) { - void *p = vmemmap_alloc_pte(pfn, node, altmap); - - if (!p) - return NULL; - ptpfn = PHYS_PFN(__pa(p)); - } else { - /* - * When a PTE/PMD entry is freed from the init_mm - * there's a free_pages() call to this page allocated - * above. Thus this get_page() is paired with the - * put_page_testzero() on the freeing path. - * This can only called by certain ZONE_DEVICE path, - * and through vmemmap_populate_compound_pages() when - * slab is available. - */ - if (slab_is_available()) - get_page(pfn_to_page(ptpfn)); - } - entry = pfn_pte(ptpfn, PAGE_KERNEL); + if (!p) + return NULL; + + entry = pfn_pte(PHYS_PFN(__pa(p)), PAGE_KERNEL); set_pte_at(&init_mm, addr, pte, entry); } else if (WARN_ON_ONCE(vmemmap_optimizable_pfn(pfn))) return NULL; -- 2.54.0