Re: [PATCH v4 0/2] kho: support preserving high-order non-compound pages

[Pranjal Shrivastava <praan@google.com>]

On Thu, Aug 13, 2026 at 02:39:46PM +0300, Mike Rapoport wrote:
> On Wed, Aug 12, 2026 at 03:46:20PM +0200, Pratyush Yadav wrote:
> > On Wed, Aug 12 2026, Pranjal Shrivastava wrote:
> > 
> > > On Wed, Aug 12, 2026 at 12:32:25PM +0200, Pratyush Yadav wrote:
> > >> On Tue, Aug 11 2026, Mike Rapoport wrote:
> > >> 
> > >> > Hi Pranjal,
> > >> >
> > >> > On Mon, Aug 03, 2026 at 11:39:41AM +0000, Pranjal Shrivastava wrote:
> > >> >> Introduction
> > >> >> ============
> > >> >> This series is required for the ongoing effort to preserve DMA allocations
> > >> >> across KHO [1]. It addresses a fundamental mismatch between the current KHO
> > >> >> restoration logic and the physical reality of high-order buddy allocations.
> > >> >
> > >> > I skimmed through the patches, they look fine to me before the in-depth
> > >> > review :)
> > >> >
> > >> > But we are really close to the merge window, so we'll anyway need to
> > >> > reiterate after v7.3-rc1.
> > >> >  
> > >> >> The Problem
> > >> >> ===========
> > >> >> The current KHO restore implementation treats all multi-page blocks as
> > >> >> split pages during restoration. Specifically, kho_restore_pages()
> > >> >> initializes every 4KB sub-page with a refcount of 1.
> > >> >> 
> > >> >> However, many kernel subsystems, most notably the DMA allocator (via
> > >> >> dma_alloc_coherent), frequently return high-order non-compound pages.
> > >> >> In this state, only the head page carries a refcount of 1, while
> > >> >> all tail pages have a refcount of 0.
> > >> >
> > >> > This hints that these patches could be a part of the DMA preservation
> > >> > series, unless you expect other users of the new API.
> > >> >
> > >> > Generally, we don't merge new APIs without the users and if DMA
> > >> > preservation is the only user, it's better to fold these two patches there.
> > >> 
> > >> Makes sense I think. We can review the patches here, but then they can
> > >> go in with the DMA series.
> > >> 
> > >
> > > So.. IIUC, I'll post a v5 here as a standalone series till we get
> > > consensus, and finally the reviewed patches can be folded with the DMA
> > > series?
> > 
> > That sounds good to me at least.
> 
> Yes, although when the patches will be a part of the DMA series we might
> notices something else :)
>  

Ohh yes, definitely! Just asking to know what to follow for the next version :)

> > -- 
> > Regards,
> > Pratyush Yadav
> 
> -- 
> Sincerely yours,
> Mike.

Thanks,
Praan