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 20:43, Kiryl Shutsemau <kas@kernel.org> wrote:
> 
> On Thu, Jan 22, 2026 at 07:42:47PM +0800, Muchun Song wrote:
>> 
>> 
>>>> On Jan 22, 2026, at 19:33, Muchun Song <muchun.song@linux.dev> wrote:
>>> 
>>> 
>>> 
>>>> 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)?
> 
> True.
> 
>> Suppose we are more aggressive. If the alignment requirement of struct page
>> cannot meet the needs of 2GB pages (which is an uncommon memory allocation
>> requirement), then users might not care about such a warning message after
>> the system boots. And if there is no allocation of pages greater than or
>> equal to 2GB for a period of time in the future, the system will have no
>> problems. But once some path allocates pages greater than or equal to 2GB,
>> the system will go into chaos. And by that time, the system log may no
>> longer have this warning message. Is that not the case?
> 
> It is.
> 
> I expect the warning to be reported early if we have configurations that
> do not satisfy the alignment requirement even in absence of the crash.

If you’re saying the issue was only caught during
testing, keep in mind that with KASLR enabled the
warning is triggered at run-time; you can’t assume it
will never appear in production.

So I don't think the administrator will notice a
warning in practice. 

> 
> Adding a check to the allocation path if way too high price for a
> theoretical problem.

Not theoretical. It’s a problem indeed. We should
 make the system works properly in production. And
 I don’t think it’s too high price, as I said only adding
 the check to CMA not buddy allocator, CMA
 allocation is not supposed to be in a hot path 
(Please carefully read the reply plan I gave at the
 very beginning.
). 

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