Re: [RFC PATCH 0/4] kho: Support preserving unsplit high-order pages

[Samiullah Khawaja]

On Wed, Jul 08, 2026 at 05:05:20PM +0000, Pranjal Shrivastava wrote:
> On Wed, Jul 08, 2026 at 04:36:46PM +0000, Samiullah Khawaja wrote:
> > On Wed, Jul 08, 2026 at 04:11:04PM +0200, Pratyush Yadav wrote:
> > > On Fri, Jul 03 2026, Pranjal Shrivastava wrote:
> > >
> >
> > [snip]
> > > >
> > > > 3. kho_restore_page() applies the correct refcount pattern based on the
> > > >    preserved metadata.
> > >
> > > Why do you need to save the type of pages in KHO metadata? For example,
> > > for pages or folios, we don't store any type information and leave it to
> > > the caller choose the right API. So reserve-mem and kho vmalloc need
> > > pages, they can call kho_{preserve,restore}_pages(), and memfd needs
> > > folios so it can call kho_{preserve,restore}_folio(). The radix tree
> > > itself does not hold the information. The caller knows what its memory
> > > is supposed to be so it calls the right restore API.
> > >
> > > So why can't we add a kho_{preserve,restore}_page_multi() (pick a better
> > > name; we can argue about the naming later)? Then your driver knows it is
> > > restoring DMA buffers so it can call kho_restore_page_multi(), and KHO
> > > takes care of initializing the pages with the right refcounts.
> > >
> > > You won't have to muck about with the ABI in that case.
> >
> > +1
> >
> > I think this makes sense. The mm already relies on the allocator to
> > track the type of pages it has and expectes it to use free_pages or
> > put_page() or free_page(), so it is natural for KHO to rely on the
> > caller to call the right restore API.
> >
> > Lets add kho_preserve/restore_page_contig|_order|_nonsplit and the dma
> > preservation can use the appropriate one during restore.
>
> Ack. I like kho_restore_contig, I don't think we'll need a preserve for
> this though? Preserve doesn't seem to be preserving refcounts, I guess
> we could rely on kho_preserve_pages and the caller can use
> kho_restore_pages_contig() to set refcount correctly for unsplit pages?

I think for consistency we can add both, even if preserve doesn't do
anything special internally.
> Thanks
> Praan

Sami