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

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

> On Jan 23, 2026, at 01:59, Kiryl Shutsemau <kas@kernel.org> wrote:
> 
> On Thu, Jan 22, 2026 at 10:02:24PM +0800, Muchun Song wrote:
>> 
>> 
>>> 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.
> 
> Let's look at what architectures actually do with vmemmap.
> 
> On 64-bit machines, we want vmemmap to be naturally aligned to
> accommodate 16GiB pages.
> 
> Assuming 64 byte struct page, it requires 256 MiB alignment for 4K
> PAGE_SIZE, 64MiB for 16K PAGE_SIZE and 16MiB for 64K PAGE_SIZE.
> 
> Only 3 architectures support HVO (select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP):
> loongarch, riscv and x86. We should make the feature conditional to HVO
> to limit exposure.
> 
> I am not sure why arm64 is not in the club.
> 
> x86 aligns vmemmap to 1G - OK.
> 
> loongarch aligns vmemmap to PMD_SIZE does not fit us with 4K and 16K
> PAGE_SIZE. It should be easily fixable. No KALSR.
> 
> riscv aligns vmemmap to section size (128MiB) which is not enough.
> Again, easily fixable.
> 

OK. After we fix all problems, I think changing WARN_ON to BUG_ON is fine.

> -- 
>  Kiryl Shutsemau / Kirill A. Shutemov