linux-kernel - Re: [PATCH RFC 10/35] mm/hugetlb: cleanup hugetlb_folio_init_tail

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Message-ID: <9156d191-9ec4-4422-bae9-2e8ce66f9d5e@redhat.com>
Date: Fri, 22 Aug 2025 07:09:17 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
 Dennis Zhou <dennis@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
 linux-ide@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mmc@...r.kernel.org, linux-mm@...ck.org,
 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linux-scsi@...r.kernel.org, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Marco Elver <elver@...gle.com>, Marek Szyprowski <m.szyprowski@...sung.com>,
 Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>, netdev@...r.kernel.org,
 Oscar Salvador <osalvador@...e.de>, Peter Xu <peterx@...hat.com>,
 Robin Murphy <robin.murphy@....com>, Suren Baghdasaryan <surenb@...gle.com>,
 Tejun Heo <tj@...nel.org>, virtualization@...ts.linux.dev,
 Vlastimil Babka <vbabka@...e.cz>, wireguard@...ts.zx2c4.com, x86@...nel.org,
 Zi Yan <ziy@...dia.com>
Subject: 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@...hat.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