[PATCH] mm/huge_memory: use folio_dup_file_rmap_pud() in copy_huge_pud()

[Konstantin Khorenko]

copy_huge_pud() adds the child's rmap of a device DAX PUD with
folio_add_file_rmap_pud(). That helper maps a folio afresh: it updates
NR_FILE_MAPPED when the folio becomes mapped and mlocks it in a
VM_LOCKED VMA. Neither applies at fork: the parent keeps the folio mapped
under the page table lock, and dup_mmap() clears VM_LOCKED in the
child's VMA.

copy_present_ptes() duplicates the rmap of a PTE with
folio_dup_file_rmap_ptes(), which only raises the mapcounts. Add
folio_dup_file_rmap_pud() for the PUD level and use it in copy_huge_pud().

No functional change.

Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: Konstantin Khorenko <khorenko@xxxxxxxxxxxxx>
---
David's fix for the device DAX PUD fork bug, commit d90bdc02e36e
("mm/huge_memory: properly handle refcounted PUD mappings on fork"),
went in before v2 of my series for the same bug [1].

This is the follow-up David asked for [2].

The Reviewed-by tags are from v2 patch 1, whose rmap.h hunk and
copy_huge_pud() line this patch carries unchanged.

The selftest from v2 [3] is not included, as it needs a real devdax
device. It is there for anyone who wants to run it.

Based on thp-7.5.misc of the mm/core tree.

[1] https://lore.kernel.org/all/20261105-vstor-150143-copy_huge_pud-folio-refcount-v2-0-2828fe5a51ed@xxxxxxxxxxxxx/
[2] https://lore.kernel.org/all/6f035f97-8b55-47ad-bd52-833b86b8ae8f@xxxxxxxxxx/
[3] https://lore.kernel.org/all/20261105-vstor-150143-copy_huge_pud-folio-refcount-v2-2-2828fe5a51ed@xxxxxxxxxxxxx/

Assisted-by: LLM
---
 include/linux/rmap.h | 22 ++++++++++++++++++++++
 mm/huge_memory.c     |  2 +-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 74cca0e3c7264..52fff8dc74213 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -564,6 +564,28 @@ static inline void folio_dup_file_rmap_pmd(struct folio *folio,
 #endif
 }
 
+/**
+ * folio_dup_file_rmap_pud - duplicate a PUD mapping of a page range of a folio
+ * @folio:	The folio to duplicate the mapping of
+ * @page:	The first page to duplicate the mapping of
+ * @dst_vma:	The destination vm area
+ *
+ * The page range of the folio is defined by [page, page + HPAGE_PUD_NR)
+ *
+ * The caller needs to hold the page table lock.
+ */
+static inline void folio_dup_file_rmap_pud(struct folio *folio,
+		struct page *page, struct vm_area_struct *dst_vma)
+{
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \
+	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
+	__folio_dup_file_rmap(folio, page, HPAGE_PUD_NR, dst_vma,
+			      PGTABLE_LEVEL_PUD);
+#else
+	WARN_ON_ONCE(true);
+#endif
+}
+
 static __always_inline int __folio_try_dup_anon_rmap(struct folio *folio,
 		struct page *page, int nr_pages, struct vm_area_struct *dst_vma,
 		struct vm_area_struct *src_vma, enum pgtable_level level)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d0e19d9e6b651..13caf81cb51af 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2100,7 +2100,7 @@ int copy_huge_pud(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
 		 * TODO: once we support anonymous pages, use
 		 * folio_try_dup_anon_rmap_*() and split if duplicating fails.
 		 */
-		folio_add_file_rmap_pud(folio, page, dst_vma);
+		folio_dup_file_rmap_pud(folio, page, dst_vma);
 		add_mm_counter(dst_mm, mm_counter_file(folio), HPAGE_PUD_NR);
 	}
 

---
base-commit: 31ea5515bf595046effc74a5bafdcc26320d2d86
change-id: 20261106-khorenko-vstor-151183-copy_huge_pud-dup-rmap-820094c56b90

Best regards,
--  
Konstantin Khorenko,
Virtuozzo Linux Kernel Team