linux-kernel - Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has

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Message-Id: <B4674CF0-7534-462B-A761-D9124234BFE4@linux.dev>
Date: Thu, 29 Jan 2026 11:00:07 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Matthew Wilcox <willy@...radead.org>,
 Usama Arif <usamaarif642@...il.com>,
 Frank van der Linden <fvdl@...gle.com>,
 Oscar Salvador <osalvador@...e.de>,
 Mike Rapoport <rppt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Zi Yan <ziy@...dia.com>,
 Baoquan He <bhe@...hat.com>,
 Michal Hocko <mhocko@...e.com>,
 Johannes Weiner <hannes@...xchg.org>,
 Jonathan Corbet <corbet@....net>,
 Huacai Chen <chenhuacai@...nel.org>,
 WANG Xuerui <kernel@...0n.name>,
 Palmer Dabbelt <palmer@...belt.com>,
 Paul Walmsley <paul.walmsley@...ive.com>,
 Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>,
 kernel-team@...a.com,
 linux-mm@...ck.org,
 linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org,
 loongarch@...ts.linux.dev,
 linux-riscv@...ts.infradead.org
Subject: Re: [PATCHv5 09/17] mm/sparse: Check memmap alignment for
 compound_info_has_mask()



> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@...nel.org> wrote:
> 
> If page->compound_info encodes a mask, it is expected that vmemmap to be
> naturally aligned to the maximum folio size.
> 
> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
> 
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> Acked-by: Zi Yan <ziy@...dia.com>
> ---
> mm/sparse.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index b5b2b6f7041b..9c0f4015778c 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -600,6 +600,19 @@ void __init sparse_init(void)
> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
> 	memblocks_present();
> 
> + 	if (compound_info_has_mask()) {
> + 		unsigned long alignment;
> + 		bool aligned;
> +
> + 		alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
> + 		aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
> +
> + 		if (IS_ENABLED(CONFIG_DEBUG_VM))
> + 			BUG_ON(!aligned);
> + 		else
> + 			WARN_ON(!aligned);

Since you’ve fixed all the problematic architectures, I don’t believe
we’ll ever hit the WARN or BUG here anymore.

I think we can now simplify the code further and just use VM_BUG_ON:
if any architecture changes in the future, the misalignment will be
caught during testing, so we won’t need to worry about it at run-time.

> + 	}
> +
> 	pnum_begin = first_present_section_nr();
> 	nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
> 
> -- 
> 2.51.2
>