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

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Jan 21, 2026 at 12:58:36PM -0500, Zi Yan wrote:
> On 21 Jan 2026, at 11:22, Kiryl Shutsemau 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)));
> > +	}
> > +
> 
> 16GB is only possible in arm64 with 64KB base page. Would it be overkill
> to align vmemmap to it unconditionally? Or how likely will this cause
> false positive warning?

CMA can give you 16GiB page on x86.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov