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

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Message-ID: <mqvgofzudkucibhyr7gsjgtb47rjibqkqce3wdd62p6kqzosxb@tv4fkzewgkt2>
Date: Mon, 22 Dec 2025 14:52:19 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>, 
	Wilcox <willy@...radead.org>
Cc: Muchun Song <muchun.song@...ux.dev>, 
	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 Mon, Dec 22, 2025 at 03:18:29PM +0100, David Hildenbrand (Red Hat) 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?

It depends on memory model and whether the arch has KASLR for memmap.

> 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,

Willy, what is timeline here?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov