linux-kernel - Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment

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Message-ID: <4ctnfkw5zqneume6px2wqgdgusfdricr4uuonv7bfjheknrt2g@fdb5ceiiar4q>
Date: Mon, 22 Dec 2025 14:02:18 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: Muchun Song <muchun.song@...ux.dev>
Cc: 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>, kernel-team@...a.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...nel.org>, 
	Matthew Wilcox <willy@...radead.org>, Usama Arif <usamaarif642@...il.com>, 
	Frank van der Linden <fvdl@...gle.com>
Subject: Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment

On Mon, Dec 22, 2025 at 04:34:40PM +0800, Muchun Song wrote:
> 
> 
> On 2025/12/18 23:09, Kiryl Shutsemau wrote:
> > The upcoming changes in compound_head() require memmap to be naturally
> > aligned to the maximum folio size.
> > 
> > Add a warning if it is not.
> > 
> > A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the
> > kernel is still likely to be functional if this strict check fails.
> 
> Different architectures default to 2 MB alignment (mainly to
> enable huge mappings), which only accommodates folios up to
> 128 MB. Yet 1 GB huge pages are still fairly common, so
> validating 16 GB (MAX_FOLIO_SIZE) alignment seems likely to
> miss the most frequent case.

I don't follow. 16 GB check is more strict that anything smaller.
How can it miss the most frequent case?

> I’m concerned that this might plant a hidden time bomb: it
> could detonate at any moment in later code, silently triggering
> memory corruption or similar failures. Therefore, I don’t
> think a WARNING is a good choice.

We can upgrade it BUG_ON(), but I want to understand your logic here
first.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov