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

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Message-ID: <3b758468-9985-49b8-948a-e5837decf52d@kernel.org>
Date: Mon, 22 Dec 2025 15:18:29 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...nel.org>, Muchun Song <muchun.song@...ux.dev>,
 Matthew Wilcox <willy@...radead.org>
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>,
 Usama Arif <usamaarif642@...il.com>, Frank van der Linden <fvdl@...gle.com>
Subject: Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment

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,

-- 
Cheers

David