Re: [LSF/MM/BPF TOPIC] 64k (or 16k) base page size on x86

[Kiryl Shutsemau <kas@kernel.org>]

On Thu, Feb 19, 2026 at 09:30:36AM -0800, Dave Hansen wrote:
> On 2/19/26 07:08, Kiryl Shutsemau wrote:
> ...
> > The patchset is large:
> > 
> >  378 files changed, 3348 insertions(+), 3102 deletions(-)
> 
> A few notes about the diffstats:
> 
> $ git diff v6.17..HEAD arch/x86 | diffstat | tail -1
>  105 files changed, 874 insertions(+), 843 deletions(-)
> $ git diff v6.17..HEAD mm | diffstat | tail -1
>  53 files changed, 1136 insertions(+), 1069 deletions(-)
> 
> The vast, vast majority of this seems to be the renames. Stuff like:
> 
> > -               new = round_down(new, PAGE_SIZE);
> > +               new = round_down(new, PTE_SIZE);
> 
> or even less worrying:
> 
> > -int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid);
> > +int set_direct_map_valid_noflush(struct page *page, unsigned numpages, bool valid);
> 
> That stuff obviously needs to be audited but it's far less concerning
> than the logic changes.
> 
> So just for review sanity, if you go forward with this, I'd very much
> appreciate a strong separation of the purely mechanical bits from any
> logic changes.

That's the plan. That's the only way I can keep myself sane :P

> > On x86, page tables are allocated from the buddy allocator and if PG_SIZE
> > is greater than 4 KB, we need a way to pack multiple page tables into a
> > single page. We could use the slab allocator for this, but it would
> > require relocating the page-table metadata out of struct page.
> 
> Others mentioned this, but I think this essentially gates what you are
> doing behind a full tree conversion over to ptdescs.

I have not followed ptdescs closely. Need to catch up.

For PoC, I will just waste full order-0 page for page table. Packing is
not required for correctness.

> The most useful thing we can do with this series is look at it and
> decide what _other_ things need to get done before the tree could
> possibly go in that direction, like ptdesc or a the disambiguation
> between PTE_SIZE and PG_SIZE that you've kicked off here.

Right.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov