Re: [PATCH RFC 10/35] mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()

On 8/21/25 23:06, David Hildenbrand wrote:

All pages were already initialized and set to PageReserved() with a refcount of 1 by MM init code.

Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to initialize struct pages?

In fact, by using __init_single_page(), we will be setting the refcount to 1 just to freeze it again immediately afterwards.

So drop the __init_single_page() and use __ClearPageReserved() instead. Adjust the comments to highlight that we are dealing with an open-coded prep_compound_page() variant.

Further, as we can now safely iterate over all pages in a folio, let's avoid the page-pfn dance and just iterate the pages directly.

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.

Signed-off-by: David Hildenbrand david@redhat.com

mm/hugetlb.c | 21 ++++++++++----------- 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c index d12a9d5146af4..ae82a845b14ad 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -3235,17 +3235,14 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio, unsigned long start_page_number, unsigned long end_page_number) {

• enum zone_type zone = zone_idx(folio_zone(folio));
• int nid = folio_nid(folio);
• unsigned long head_pfn = folio_pfn(folio);
• unsigned long pfn, end_pfn = head_pfn + end_page_number;

• struct page *head_page = folio_page(folio, 0);
• struct page *page = folio_page(folio, start_page_number);
• unsigned long i; int ret;

• for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {

• struct page *page = pfn_to_page(pfn);
  

• __init_single_page(page, pfn, zone, nid);
  

• prep_compound_tail((struct page *)folio, pfn - head_pfn);
  

• for (i = start_page_number; i < end_page_number; i++, page++) {

• __ClearPageReserved(page);
  

• prep_compound_tail(head_page, i);
  

  ret = page_ref_freeze(page, 1); VM_BUG_ON(!ret); }

@@ -3257,12 +3254,14 @@ 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 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));

}

--Mika