[PATCHv5 09/17] mm/sparse: Check memmap alignment for compound_info_has_mask()

[Muchun Song]

> On Jan 29, 2026, at 11:10, Zi Yan <ziy at nvidia.com> wrote:
> 
> On 28 Jan 2026, at 22:00, Muchun Song wrote:
> 
>>> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas at kernel.org> wrote:
>>> 
>>> If page->compound_info encodes a mask, it is expected that vmemmap to be
>>> naturally aligned to the maximum folio size.
>>> 
>>> Trigger a BUG() for CONFIG_DEBUG_VM=y or WARN() otherwise.
>>> 
>>> Signed-off-by: Kiryl Shutsemau <kas at kernel.org>
>>> Acked-by: Zi Yan <ziy at nvidia.com>
>>> ---
>>> mm/sparse.c | 13 +++++++++++++
>>> 1 file changed, 13 insertions(+)
>>> 
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index b5b2b6f7041b..9c0f4015778c 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -600,6 +600,19 @@ void __init sparse_init(void)
>>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
>>> memblocks_present();
>>> 
>>> +  if (compound_info_has_mask()) {
>>> +  	unsigned long alignment;
>>> +  	bool aligned;
>>> +
>>> +  	alignment = MAX_FOLIO_NR_PAGES * sizeof(struct page);
>>> +  	aligned = IS_ALIGNED((unsigned long) pfn_to_page(0), alignment);
>>> +
>>> +  	if (IS_ENABLED(CONFIG_DEBUG_VM))
>>> +  		BUG_ON(!aligned);
>>> +  	else
>>> +  		WARN_ON(!aligned);
>> 
>> Since you’ve fixed all the problematic architectures, I don’t believe
>> we’ll ever hit the WARN or BUG here anymore.
>> 
>> I think we can now simplify the code further and just use VM_BUG_ON:
>> if any architecture changes in the future, the misalignment will be
>> caught during testing, so we won’t need to worry about it at run-time.
>> 
> 
> VM_WARN_ON should be sufficient, since bots should report warnings
> from any patch/change.

I’m not sure a WARN will get developers’ attention, since the message
is unlikely to have any visible consequences and only fires on
allocations with a special order.

> 
>>> +  }
>>> +
>>> pnum_begin = first_present_section_nr();
>>> nid_begin = sparse_early_nid(__nr_to_section(pnum_begin));
>>> 
>>> -- 
>>> 2.51.2
>>> 
> 
> 
> Best Regards,
> Yan, Zi