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

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* David Hildenbrand <david@redhat.com> [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 <david@redhat.com>

Acked-by: Liam R. Howlett <Liam.Howlett@oracle.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
>