Re: [PATCHv6 06/17] LoongArch/mm: Align vmemmap to maximal folio size

[David Hildenbrand (Arm)]

On 2/4/26 17:56, David Hildenbrand (arm) wrote:
> On 2/2/26 16:56, Kiryl Shutsemau wrote:
> > The upcoming change to the HugeTLB vmemmap optimization (HVO) requires
> > struct pages of the head page to be naturally aligned with regard to the
> > folio size.
> >
> > Align vmemmap to MAX_FOLIO_NR_PAGES.
> >
> > Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> > ---
> >   arch/loongarch/include/asm/pgtable.h | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/ 
> > include/asm/pgtable.h
> > index c33b3bcb733e..f9416acb9156 100644
> > --- a/arch/loongarch/include/asm/pgtable.h
> > +++ b/arch/loongarch/include/asm/pgtable.h
> > @@ -113,7 +113,8 @@ extern unsigned long empty_zero_page[PAGE_SIZE / 
> > sizeof(unsigned long)];
> >        min(PTRS_PER_PGD * PTRS_PER_PUD * PTRS_PER_PMD * PTRS_PER_PTE * 
> > PAGE_SIZE, (1UL << cpu_vabits) / 2) - PMD_SIZE - VMEMMAP_SIZE - 
> > KFENCE_AREA_SIZE)
> >   #endif
> > -#define vmemmap        ((struct page *)((VMALLOC_END + PMD_SIZE) & 
> > PMD_MASK))
> > +#define VMEMMAP_ALIGN    max(PMD_SIZE, MAX_FOLIO_NR_PAGES * 
> > sizeof(struct page))
> > +#define vmemmap        ((struct page *)(ALIGN(VMALLOC_END, 
> > VMEMMAP_ALIGN)))
>
>
> Same comment, the "MAX_FOLIO_NR_PAGES * sizeof(struct page)" is just 
> black magic here
> and the description of the situation is wrong.
>
> Maybe you want to pull the magic "MAX_FOLIO_NR_PAGES * sizeof(struct 
> page)" into the core and call it
>
> #define MAX_FOLIO_VMEMMAP_ALIGN    (MAX_FOLIO_NR_PAGES * sizeof(struct 
> page))
>
> But then special case it base on (a) HVO being configured in an (b) HVO 
> being possible
>
> #ifdef HUGETLB_PAGE_OPTIMIZE_VMEMMAP && is_power_of_2(sizeof(struct page)
> /* A very helpful comment explaining the situation. */
> #define MAX_FOLIO_VMEMMAP_ALIGN    (MAX_FOLIO_NR_PAGES * sizeof(struct 
> page))
> #else
> #define MAX_FOLIO_VMEMMAP_ALIGN    0
> #endif
>
> Something like that.

Thinking about this ...

the vmemmap start is always struct-page-aligned. Otherwise we'd be in 
trouble already.

Isn't it then sufficient to just align the start to MAX_FOLIO_NR_PAGES?

Let's assume sizeof(struct page) == 64 and MAX_FOLIO_NR_PAGES = 512 for 
simplicity.

vmemmap start would be multiples of 512 (0x0010000000).

512, 1024, 1536, 2048 ...

Assume we have an 256-pages folio at 1536+256 = 0x111000000

Assume we have the last page of that folio (0x011111111111), we would 
just get to the start of that folio by AND-ing with ~(256-1).

Which case am I ignoring?

--
Cheers,

David