On Thu, Aug 28, 2025 at 12:01:17AM +0200, David Hildenbrand wrote:
> 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,
> 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.
>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
> ---
> mm/hugetlb.c | 20 ++++++++++++--------
> 1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4a97e4f14c0dc..1f42186a85ea4 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);
>
> + /*
> + * We mark all tail pages with memblock_reserved_mark_noinit(),
> + * so these pages are completely uninitialized.
^ not? ;-)
> + */
> + 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
>
--
Sincerely yours,
Mike.
