[PATCH v1 13/36] mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()

[David Hildenbrand]

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.
+        */
+       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