Re: [PATCH v1 13/36] mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()
From: Liam R. Howlett <hidden>
Date: 2025-08-29 14:58:53
Also in:
dri-devel, intel-gfx, io-uring, kvm, linux-arm-kernel, linux-crypto, linux-ide, linux-iommu, linux-kselftest, linux-mips, linux-mm, linux-mmc, linux-riscv, linux-s390, linux-scsi, lkml, netdev
* David Hildenbrand [off-list ref] [250827 18:06]:
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 <redacted>
Acked-by: Liam R. Howlett <redacted>
quoted hunk ↗ jump to hunk
--- 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