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

[Kiryl Shutsemau <kas@kernel.org>]

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.

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

-- 
  Kiryl Shutsemau / Kirill A. Shutemov