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

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Message-Id: <CDAEC896-E3EB-4EAB-9F0F-70BC448B3B9A@linux.dev>
Date: Mon, 22 Dec 2025 22:55:16 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: David Hildenbrand <david@...nel.org>
Cc: Kiryl Shutsemau <kas@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 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>,
 Usama Arif <usamaarif642@...il.com>,
 Frank van der Linden <fvdl@...gle.com>
Subject: Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment



> On Dec 22, 2025, at 22:18, David Hildenbrand (Red Hat) <david@...nel.org> wrote:
> 
> On 12/22/25 15:02, Kiryl Shutsemau wrote:
>>> 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.
> 
> Definitely no BUG_ON(). I would assume this is something we would find early during testing, so even a VM_WARN_ON_ONCE() should be good enough?
> 
> This smells like a possible problem, though, as soon as some architecture wants to increase the folio size. What would be the expected step to ensure the alignment is done properly?
> 
> But OTOH, as I raised Willy's work will make all of that here obsolete either way, so maybe not worth worrying about that case too much,

Hi David,

I hope you're doing well. I must admit I have limited knowledge of Willy's work, and I was wondering if you might be kind enough to share any publicly available links where I could learn more about the future direction of this project. I would be truly grateful for your guidance.
Thank you very much in advance.

Best regards,

> 
> --
> Cheers
> 
> David