Re: [PATCHv4 07/14] mm/sparse: Check memmap alignment for compound_info_has_mask()

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

> On Jan 22, 2026, at 19:28, Kiryl Shutsemau <kas@kernel.org> wrote:
> 
> On Thu, Jan 22, 2026 at 11:10:26AM +0800, Muchun Song wrote:
>> 
>> 
>>> On Jan 22, 2026, at 00:22, Kiryl Shutsemau <kas@kernel.org> wrote:
>>> 
>>> If page->compound_info encodes a mask, it is expected that 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.
>>> 
>>> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
>>> ---
>>> include/linux/mmzone.h | 1 +
>>> mm/sparse.c            | 5 +++++
>>> 2 files changed, 6 insertions(+)
>>> 
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index 390ce11b3765..7e4f69b9d760 100644
>>> --- a/include/linux/mmzone.h
>>> +++ b/include/linux/mmzone.h
>>> @@ -91,6 +91,7 @@
>>> #endif
>>> 
>>> #define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER)
>>> +#define MAX_FOLIO_SIZE (PAGE_SIZE << MAX_FOLIO_ORDER)
>>> 
>>> enum migratetype {
>>> MIGRATE_UNMOVABLE,
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index 17c50a6415c2..5f41a3edcc24 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -600,6 +600,11 @@ void __init sparse_init(void)
>>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>>> memblocks_present();
>>> 
>>> +  if (compound_info_has_mask()) {
>>> +  WARN_ON(!IS_ALIGNED((unsigned long)pfn_to_page(0),
>>> +     MAX_FOLIO_SIZE / sizeof(struct page)));
>> 
>> I still have concerns about this. If certain architectures or configurations,
>> especially when KASLR is enabled, do not meet the requirements during the
>> boot stage, only specific folios larger than a certain size might end up with
>> incorrect struct page entries as the system runs. How can we detect issues
>> arising from either updating the struct page or making incorrect logical
>> judgments based on information retrieved from the struct page?
>> 
>> After all, when we see this warning, we don't know when or if a problem will
>> occur in the future. It's like a time bomb in the system, isn't it? Therefore,
>> I would like to add a warning check to the memory allocation place, for
>> example: 
>> 
>>  WARN_ON(!IS_ALIGNED((unsigned long)&folio->page, folio_size / sizeof(struct page)));
> 
> I don't think it is needed. Any compound page usage would trigger the
> problem. It should happen pretty early.

Why would you think it would be discovered early? If the alignment of struct page
can only meet the needs of 4M pages (i.e., the largest pages that buddy can
allocate), how can you be sure that there will be a similar path using CMA
early on if the system allocates through CMA in the future (after all, CMA
is used much less than buddy)?

> 
> -- 
>  Kiryl Shutsemau / Kirill A. Shutemov