Re: [PATCH v3 08/17] mm/sparse-vmemmap: support section-based vmemmap optimization
From: Muchun Song
Date: Mon Aug 10 2026 - 23:54:56 EST
On 2026/8/4 11:55, Muchun Song wrote:
Teach sparse-vmemmap population code to use the compound page order
when deciding whether a vmemmap page can be optimized.
With this information, the common sparse-vmemmap population path can
allocate or reuse shared tail vmemmap pages directly instead of relying
on HugeTLB-specific handling.
This centralizes vmemmap optimization logic in the sparse-vmemmap code,
based on section metadata, and prepares for sharing the same mechanism
across different users of vmemmap optimization, including HugeTLB and
DAX.
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
v2:
- Keep vmemmap accounting and population logic in sparse-vmemmap.c
(suggested by Mike Rapoport)
- Move vmemmap_get_tail() before its first use instead of adding only a
forward declaration in the previous patch (suggested by Mike Rapoport)
- Simplify the PMD path handling for HVO-covered sections
---
mm/sparse-vmemmap.c | 36 ++++++++++++++++++++++++++++++------
mm/sparse.c | 4 ++--
mm/sparse.h | 7 +++++++
3 files changed, 39 insertions(+), 8 deletions(-)
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index b770fe2428fd..b69a7af76858 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -186,6 +186,11 @@ static __meminit struct page *vmemmap_get_tail(unsigned int order, struct zone *
return tail;
}
Sashiko said there is a data race for store to zone->vmemmap_tails[idx]
between hugetlb_vmemmap.c and here. This is another false positive. There
couldn't be any concurrency during system startup, and after startup,
since HVO cannot be applied via memory hotplug at this time, there isn't
any either.
The other two reports are also based on issues that occur during the memory
hotplug phase, so they are false positives as well.
Muchun,
Thanks.
+#else
+static inline struct page *vmemmap_get_tail(unsigned int order, struct zone *zone)
+{
+ return NULL;
+}
#endif
static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
@@ -193,12 +198,24 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
unsigned long ptpfn, unsigned long flags)
{
pte_t *pte = pte_offset_kernel(pmd, addr);
+ unsigned long pfn = page_to_pfn((struct page *)addr);
+
if (pte_none(ptep_get(pte))) {
pte_t entry;
- void *p;
+
+ if (pfn_vmemmap_optimizable(pfn) && ptpfn == (unsigned long)-1) {
+ unsigned int order = pfn_to_section_order(pfn);
+ struct zone *zone = pfn_to_zone(pfn, node);
+ struct page *page = vmemmap_get_tail(order, zone);
+
+ if (!page)
+ return NULL;
+ ptpfn = page_to_pfn(page);
+ }
if (ptpfn == (unsigned long)-1) {
- p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
+ void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
+
if (!p)
return NULL;
ptpfn = PHYS_PFN(__pa(p));
@@ -217,7 +234,8 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
}
entry = pfn_pte(ptpfn, PAGE_KERNEL);
set_pte_at(&init_mm, addr, pte, entry);
- }
+ } else if (WARN_ON_ONCE(pfn_vmemmap_optimizable(pfn)))
+ return NULL;
return pte;
}
@@ -406,6 +424,9 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
pmd_t *pmd;
for (addr = start; addr < end; addr = next) {
+ unsigned long pfn = page_to_pfn((struct page *)addr);
+ const struct mem_section *ms = __pfn_to_section(pfn);
+
next = pmd_addr_end(addr, end);
pgd = vmemmap_pgd_populate(addr, node);
@@ -421,7 +442,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
return -ENOMEM;
pmd = pmd_offset(pud, addr);
- if (pmd_none(pmdp_get(pmd))) {
+ if (pmd_none(pmdp_get(pmd)) && !section_vmemmap_optimizable(ms)) {
void *p;
p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
@@ -439,8 +460,11 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
*/
return -ENOMEM;
}
- } else if (vmemmap_check_pmd(pmd, node, addr, next))
+ } else if (vmemmap_check_pmd(pmd, node, addr, next)) {
+ if (WARN_ON_ONCE(section_vmemmap_optimizable(ms)))
+ return -ENOTSUPP;
continue;
+ }
if (vmemmap_populate_basepages(addr, next, node, altmap))
return -ENOMEM;
}
@@ -648,7 +672,7 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
}
}
-static int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
+int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
{
const struct mem_section *ms = __pfn_to_section(pfn);
diff --git a/mm/sparse.c b/mm/sparse.c
index ca9875f568d3..24555a32a5d9 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -315,8 +315,8 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
nid, NULL, NULL);
if (!map)
panic("Failed to allocate memmap for section %lu\n", pnum);
- memmap_boot_pages_add(DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
- PAGE_SIZE));
+ memmap_boot_pages_add(section_nr_vmemmap_pages(pfn, PAGES_PER_SECTION,
+ NULL, NULL));
sparse_init_early_section(nid, map, pnum, 0);
}
}
diff --git a/mm/sparse.h b/mm/sparse.h
index 6ad190ec48cf..f8f852f9f8a2 100644
--- a/mm/sparse.h
+++ b/mm/sparse.h
@@ -105,8 +105,15 @@ static inline void sparse_init(void) {}
*/
#ifdef CONFIG_SPARSEMEM_VMEMMAP
void sparse_init_subsection_map(void);
+int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
+ struct vmem_altmap *altmap, struct dev_pagemap *pgmap);
#else
static inline void sparse_init_subsection_map(void) {}
+static inline int section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
+ struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
+{
+ return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
+}
#endif /* CONFIG_SPARSEMEM_VMEMMAP */
#endif /* __MM_SPARSE_H */