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

[Zi Yan <ziy@nvidia.com>]

On 28 Jan 2026, at 22:23, Muchun Song wrote:

>> On Jan 29, 2026, at 11:10, Zi Yan <ziy@nvidia.com> wrote:
>>
>> On 28 Jan 2026, at 22:00, Muchun Song wrote:
>>
>>>> On Jan 28, 2026, at 21:54, Kiryl Shutsemau <kas@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@kernel.org>
>>>> Acked-by: Zi Yan <ziy@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.

If a developer misses the WARN and the patch gets into linux-mm or linux-next,
kernel test robot runs selftests on the kernel and reports any warnings
to the mailing list. Do we have any related test in selftests/mm? That should
help us catch anything if a developer does not catch it.

Best Regards,
Yan, Zi