[PATCH v2 13/37] mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()

David Hildenbrand
2025-09-01T15:08:07Z

We can now safely iterate over all pages in a folio, so no need for the
pfn_to_page().

Also, as we already force the refcount in __init_single_page() to 1
through init_page_count(), we can just set the refcount to 0 and
avoid page_ref_freeze() + VM_BUG_ON. Likely, in the future, we would just
want to tell __init_single_page() to which value to initialize the
refcount.

Further, adjust the comments to highlight that we are dealing with an
open-coded prep_compound_page() variant, and add another comment explaining
why we really need the __init_single_page() only on the tail pages.

Note that the current code was likely problematic, but we never ran into
it: prep_compound_tail() would have been called with an offset that might
exceed a memory section, and prep_compound_tail() would have simply
added that offset to the page pointer -- which would not have done the
right thing on sparsemem without vmemmap.

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/hugetlb.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d3542e92a712e..56e6d2af08434 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3237,17 +3237,18 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
{
enum zone_type zone = zone_idx(folio_zone(folio));
int nid = folio_nid(folio);
+ struct page *page = folio_page(folio, start_page_number);
unsigned long head_pfn = folio_pfn(folio);
unsigned long pfn, end_pfn = head_pfn + end_page_number;
- int ret;
-
- for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
- struct page *page = pfn_to_page(pfn);

+ /*
+ * As we marked all tail pages with memblock_reserved_mark_noinit(),
+ * we must initialize them ourselves here.
+ */
+ for (pfn = head_pfn + start_page_number; pfn < end_pfn; page++, pfn++) {
__init_single_page(page, pfn, zone, nid);
prep_compound_tail((struct page *)folio, pfn - head_pfn);
- ret = page_ref_freeze(page, 1);
- VM_BUG_ON(!ret);
+ set_page_count(page, 0);
}
}

@@ -3257,12 +3258,15 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
{
int ret;

- /* Prepare folio head */
+ /*
+ * This is an open-coded prep_compound_page() whereby we avoid
+ * walking pages twice by initializing/preparing+freezing them in the
+ * same go.
+ */
__folio_clear_reserved(folio);
__folio_set_head(folio);
ret = folio_ref_freeze(folio, 1);
VM_BUG_ON(!ret);
- /* Initialize the necessary tail struct pages */
hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
prep_compound_head((struct page *)folio, huge_page_order(h));
}
--
2.50.1