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

[Muchun Song <muchun.song@linux.dev>]

> On Dec 22, 2025, at 22:03, Kiryl Shutsemau <kas@kernel.org> 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?

Sorry, I didn’t make myself clear. What I meant
is that if this warning triggers, it implies the
largest-sized folio isn’t properly aligned, and
the 1 GB folios are probably mis-aligned too.
Your commit message says
“MAX_FOLIO_ORDER is very rarely used,” but
I want to stress that 1 GB folios are actually
 common. If they’re also mis-aligned, we’re
quietly planting a land-mine. That’s why I’m
worried a mere warning isn’t enough—it
leaves a latent bug in the system.

If there’s a problem, we should stop right
here—this is the earliest place where it will surface.

As David assumed, if we expect to catch the
problem during testing, then I think VM_BUG_ON
would be more appropriate.

Thanks.

> 
>> 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