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Sat, 10 Oct 2026 00:58:41 -0700 (PDT) Received: from G6L4RL2QG9.bytedance.net ([139.177.225.249]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ab57c05724sm3473569a91.4.2026.10.10.00.58.32 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Sat, 10 Oct 2026 00:58:40 -0700 (PDT) From: Muchun Song To: Madhavan Srinivasan , Mike Rapoport , Andrew Morton , David Hildenbrand Cc: Michael Ellerman , Nicholas Piggin , Christophe Leroy , Ritesh Harjani , Shrikanth Hegde , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Qi Zheng , Lance Yang , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Muchun Song , muchun.song@linux.dev Subject: [PATCH v4 2/6] mm/sparse-vmemmap: support device DAX in common vmemmap path Date: Sat, 10 Oct 2026 15:57:51 +0800 Message-ID: <20261010075755.83339-3-songmuchun@bytedance.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20261010075755.83339-1-songmuchun@bytedance.com> References: <20261010075755.83339-1-songmuchun@bytedance.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Lance Yang Acked-by: Qi Zheng --- v4: - Collect Reviewed-by from Lance Yang v3: - Collect Acked-by from Qi Zheng v2: - Expand comments around slab initialization to explain zone lookup and page refcounting (suggested by Qi Zheng) --- mm/sparse-vmemmap.c | 64 +++++++++++++++++++++++++-------------------- 1 file changed, 35 insertions(+), 29 deletions(-) diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c index b23efff80137..62327ac15206 100644 --- a/mm/sparse-vmemmap.c +++ b/mm/sparse-vmemmap.c @@ -241,18 +241,43 @@ 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. + * + * The backing page may be shared by enough PTE mappings to exhaust + * the positive range of its reference count. Stop populating the + * vmemmap if another reference cannot be acquired. + */ + if (slab_is_available() && !try_get_page(page)) + return NULL; + return page_address(page); } @@ -264,31 +289,12 @@ 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 try_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. - * - * Use try_get_page() to prevent the shared page refcount - * from overflowing. - */ - if (slab_is_available() && - !try_get_page(pfn_to_page(ptpfn))) - return NULL; - } - 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