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

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

> On Dec 23, 2025, at 17:38, David Hildenbrand (Red Hat) <david@kernel.org> wrote:
> 
> On 12/22/25 15:55, Muchun Song wrote:
>>> On Dec 22, 2025, at 22:18, David Hildenbrand (Red Hat) <david@kernel.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,
> 
> Hi! :)
> 
>> 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.
> 
> There is some information to be had at [1], but more at [2]. Take a look at [2] in "After those projects are complete - Then we can shrink struct page to 32 bytes:"
> 
> In essence, all pages (belonging to a memdesc) will have a "memdesc" pointer (that replaces the compound_head pointer).
> 
> "Then we make page->compound_head point to the dynamically allocated memdesc rather than the first page. Then we can transition to the above layout. "
> 
> The "memdesc" could be a pointer to a "struct folio" that is allocated from the slab.
> 
> So in the new memdesc world, all pages part of a folio will point at the allocated "struct folio", not the head page where "struct folio" currently overlays "struct page".
> 
> That would mean that the proposal in this patch set will have to be reverted again.
> 
> 
> At LPC, Willy said that he wants to have something out there in the first half of 2026.
> 
> [1] https://kernelnewbies.org/MatthewWilcox/Memdescs
> [2] https://kernelnewbies.org/MatthewWilcox/Memdescs/Path

Many thanks for taking the time to explain everything in detail and for providing
such valuable information. I plan to invest additional time to fully understand
the details you’ve shared.

Muchun,
Thanks.

> 
> -- 
> Cheers
> 
> David