Re: [PATCH 07/12] mm: hugetlb_vmemmap: simplify shared tail page handling

[Lance Yang <lance.yang@linux.dev>]

On Tue, Oct 20, 2026 at 07:58:19PM +0800, Muchun Song wrote:
>The remap walker currently carries a shared tail page selected by its
>caller, even though the optimize and restore callbacks can derive the
>same information from the folio being processed.  This adds redundant
>state to the walker and couples its interface to caller-side preparation.
>
>Derive the shared mapping state in the callbacks instead, keeping the
>mapping policy with the PTE being processed and simplifying the remap
>interface.
>
>No functional change intended.
>
>Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>---
> include/linux/vmemmap-optimization.h |  6 +-
> mm/hugetlb_vmemmap.c                 | 89 ++++++++++++----------------
> mm/mm_init.c                         |  6 +-
> mm/sparse-vmemmap.c                  |  5 +-
> 4 files changed, 49 insertions(+), 57 deletions(-)
>
>diff --git a/include/linux/vmemmap-optimization.h b/include/linux/vmemmap-optimization.h
>index 065840f3c241..858311a71b0c 100644
>--- a/include/linux/vmemmap-optimization.h
>+++ b/include/linux/vmemmap-optimization.h
>@@ -91,11 +91,13 @@ static inline struct page *vmemmap_shared_tail_page(unsigned int order,
> }
> #endif /* CONFIG_VMEMMAP_OPTIMIZATION */
> 
>-static inline bool vmemmap_optimizable_pfn(unsigned long pfn)
>+static inline bool vmemmap_optimizable_pfn(unsigned long pfn, unsigned int order)
> {
>-	const unsigned int order = pfn_to_section_compound_order(pfn);
> 	const unsigned long nr_pages = 1UL << order;
> 
>+	if (!IS_ENABLED(CONFIG_VMEMMAP_OPTIMIZATION))
>+		return false;
>+
> 	if (!is_power_of_2(sizeof(struct page)))
> 		return false;
> 
>diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>index 18b72d553955..8fac61b7d74a 100644
>--- a/mm/hugetlb_vmemmap.c
>+++ b/mm/hugetlb_vmemmap.c
>@@ -24,22 +24,18 @@
>  * struct vmemmap_remap_walk - walk vmemmap page table
>  *
>  * @remap_pte:		called for each lowest-level entry (PTE).
>- * @nr_walked:		the number of walked pte.
>- * @vmemmap_tail:	the page to be installed as non-first in the vmemmap range
>+ * @nr_walked:		the number of PTEs successfully remapped.
>  * @vmemmap_pages:	the list head of the vmemmap pages that can be freed
>  *			or is mapped from.
>  * @flags:		used to modify behavior in vmemmap page table walking
>  *			operations.
>  */
> struct vmemmap_remap_walk {
>-	void			(*remap_pte)(pte_t *pte, unsigned long addr,
>+	int			(*remap_pte)(pte_t *pte, unsigned long addr,
> 					     struct vmemmap_remap_walk *walk);
>-
> 	unsigned long		nr_walked;
>-	struct page		*vmemmap_tail;
> 	struct list_head	*vmemmap_pages;
> 
>-
> /* Skip the TLB flush while updating the vmemmap page tables. */
> #define VMEMMAP_REMAP_NO_TLB_FLUSH	BIT(0)
> 	unsigned long		flags;
>@@ -140,12 +136,14 @@ static int vmemmap_pmd_entry(pmd_t *pmd, unsigned long addr,
> static int vmemmap_pte_entry(pte_t *pte, unsigned long addr,
> 			     unsigned long next, struct mm_walk *walk)
> {
>+	int ret;
> 	struct vmemmap_remap_walk *vmemmap_walk = walk->private;
> 
>-	vmemmap_walk->remap_pte(pte, addr, vmemmap_walk);
>-	vmemmap_walk->nr_walked++;
>+	ret = vmemmap_walk->remap_pte(pte, addr, vmemmap_walk);
>+	if (!ret)
>+		vmemmap_walk->nr_walked++;
> 
>-	return 0;
>+	return ret;
> }
> 
> static const struct mm_walk_ops vmemmap_remap_ops = {
>@@ -213,51 +211,51 @@ static struct page *vmemmap_dup_page_from_list(const struct page *src,
> 	return dst;
> }
> 
>-static void vmemmap_optimize_pte(pte_t *pte, unsigned long addr,
>-				 struct vmemmap_remap_walk *walk)
>+static inline struct page *folio_vmemmap_shared_tail_page(const struct folio *folio)
> {
>-	struct page *page = pte_page(ptep_get(pte));
>-	pte_t entry;
>-
>-	/* Remapping the head page requires r/w */
>-	if (unlikely(walk->nr_walked == 0)) {
>-		struct page *head;
>+	return vmemmap_shared_tail_page(folio_order(folio), folio_zone(folio));
>+}
> 
>-		VM_WARN_ON_ONCE(!PageHead((const struct page *)addr));
>+static int vmemmap_optimize_pte(pte_t *pte, unsigned long addr,
>+				struct vmemmap_remap_walk *walk)
>+{
>+	pte_t entry;
>+	struct page *src = pte_page(ptep_get(pte)), *dst;
>+	const struct page *page = (const struct page *)addr;
>+	const struct folio *folio = page_folio(page);
> 
>-		head = vmemmap_dup_page_from_list(page, walk->vmemmap_pages);
>-		entry = mk_pte(head, PAGE_KERNEL);
>+	if (likely(vmemmap_optimizable_pfn(page_to_pfn(page), folio_order(folio)))) {
>+		dst = folio_vmemmap_shared_tail_page(folio);
>+		if (!dst)
>+			return -ENOMEM;

One thing caught my eye here ... shared tail allocation can now fail after
we've already remapped the head.

IIUC, nothing to worry about there, though :) since rollback keeps the
new head mapped and flushes before clearing the optimized flag or freeing
the old backing page.

[...]

Reviewed-by: Lance Yang <lance.yang@linux.dev>