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

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

> On Jan 29, 2026, at 11:29, Zi Yan <ziy@nvidia.com> wrote:
> 
> 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.

I looked at the selftest and it doesn’t seem to have a test that
allocates at MAX_FOLIO_ORDER and checks that it works correctly.

> 
> Best Regards,
> Yan, Zi