Re: [PATCH v4 1/2] kho: Introduce a helper to init high order pages

[Suren Baghdasaryan]

On Thu, Aug 13, 2026 at 4:38 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> (added Suren)

Thanks!

>
> On Wed, Aug 12, 2026 at 12:54:45PM +0200, Pratyush Yadav wrote:
> > On Mon, Aug 03 2026, Pranjal Shrivastava wrote:
> >
> > > The current KHO restoration logic assumes all multi-page blocks are
> > > split into independent 4KB pages. Break out a helper to prepare for
> > > supporting high-order non-compound pages.
> > >
> > > Extract kho_init_high_order_page() to handle the refcount pattern
> > > where only the head page is refcounted. Use the helper for folio
> > > restoration that requires a similar refcount logic.
> > >
> > > Reviewed-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
> > > Signed-off-by: Pranjal Shrivastava <praan@xxxxxxxxxx>
> > > ---
> > >  kernel/liveupdate/kexec_handover.c | 29 +++++++++++++++++++----------
> > >  1 file changed, 19 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> > > index 4834a809985a..e836efd98795 100644
> > > --- a/kernel/liveupdate/kexec_handover.c
> > > +++ b/kernel/liveupdate/kexec_handover.c
> > > @@ -357,6 +357,24 @@ int kho_radix_walk_tree(struct kho_radix_tree *tree,
> > >  }
> > >  EXPORT_SYMBOL_GPL(kho_radix_walk_tree);
> > >
> > > +/* For physically contiguous pages. */
> > > +static void kho_init_high_order_page(struct page *page, unsigned int order)
> > > +{
> > > +   unsigned long nr_pages = (1UL << order);
> > > +
> > > +   /* Head page gets refcount of 1. */
> > > +   set_page_count(page, 1);
> > > +   /* Clear head page's codetag to avoid accounting mismatch. */
> > > +   clear_page_tag_ref(page);
> > > +
> > > +   /* For high-order blocks, tail pages get a page count of zero. */
> > > +   for (unsigned long i = 1; i < nr_pages; i++) {
> > > +           set_page_count(page + i, 0);
> > > +           /* Clear each page's codetag to avoid accounting mismatch. */
> > > +           clear_page_tag_ref(page + i);
> > > +   }
> >
> > That's sneaky...
> >
> > The patch _almost_ looks like pure code movement, but then adds this
> > little change. I'm not saying this is intentionally sneaky or anything
> > of the sort, but these kind of things are easy to miss during code
> > movement and should get a patch of their own or at least be called out
> > in the commit message.
> >
> > I don't know how page tags work, but IIRC when the change was originally
> > added by Ran, he said that we don't need to clear the tag for tail
> > pages. That held true for folios, does it not hold true for non-compound
> > high-order pages?

Yeah, only the head page is tagged with number of pages of (1 <<
order), so the rest of the pages don't have tags to be cleared.

> >
> > > +}
> > > +
> > >  /* For physically contiguous 0-order pages. */
> > >  static void kho_init_pages(struct page *page, unsigned long nr_pages)
> > >  {
> > > @@ -369,16 +387,7 @@ static void kho_init_pages(struct page *page, unsigned long nr_pages)
> > >
> > >  static void kho_init_folio(struct page *page, unsigned int order)
> > >  {
> > > -   unsigned long nr_pages = (1 << order);
> > > -
> > > -   /* Head page gets refcount of 1. */
> > > -   set_page_count(page, 1);
> > > -   /* Clear head page's codetag to avoid accounting mismatch. */
> > > -   clear_page_tag_ref(page);
> > > -
> > > -   /* For higher order folios, tail pages get a page count of zero. */
> > > -   for (unsigned long i = 1; i < nr_pages; i++)
> > > -           set_page_count(page + i, 0);
> > > +   kho_init_high_order_page(page, order);
> > >
> > >     if (order > 0)
> > >             prep_compound_page(page, order);
> >
> > --
> > Regards,
> > Pratyush Yadav
>
> --
> Sincerely yours,
> Mike.