[PATCH 09/12] memblock: introduce MEMBLOCK_KHO_SCRATCH_EXT

[Pasha Tatashin]

On 05-11 18:46, Pratyush Yadav wrote:
> On Mon, May 11 2026, Mike Rapoport wrote:
> 
> > On Wed, Apr 29, 2026 at 03:39:11PM +0200, Pratyush Yadav wrote:
> >> From: "Pratyush Yadav (Google)" <pratyush at kernel.org>
> >> 
> >> In the upcoming commits, the KHO will learn how to discover free blocks
> >> of memory by walking the KHO radix tree. It will then mark those regions
> >> as scratch to allow memory allocation in case scratch runs low.
> >> 
> >> To differentiate the extended scratch areas from the main scratch areas,
> >> introduce MEMBLOCK_KHO_SCRATCH_EXT. Use it when choosing memblock flags
> >> for allocations during scratch-only. Teach should_skip_region() to check
> >> for both flags before deciding if the region should be skipped.
> >
> > Why there's a need to differentiate SCRATCH and SCRATCH_EXT?
> > SCRATCH (I still hate the name) means "memory memblock can safely use for

 +1000

I also strongly dislike this name and mentioned it in another thread
earlier today.

If we ever decide to s/scratch/something-else/ globally, that should be a
separate cleanup effort. However, since we are introducing a brand new flag
here, we can discuss a better name for the _ext portion to avoid overloading
the "scratch" concept.

> > the allocations". Initially this memory comes from the reservations in the
> > first kernel, but if the second kernel can find more memory to extend it,
> > why that additional memory should be treated differently? 
> 
> Two reasons:
> 
> 1. We mark SCRATCH as MIGRATE_CMA. We don't want to do that for
>    SCRATCH_EXT since this memory can be used for non-movable
>    allocations.
> 
> 2. Gigantic (1G) huge pages can not be allocated from scratch. They can
>    be preserved memory and thus should not be allocated from SCRATCH.
>    See patch 12 that does allocations for gigantic huge pages only from
>    SCRATCH_EXT.
> 
> I will add this in the commit message for the next version.
> 
> Naming is hard, so if you have any better names I'm all ears :-)

IMO, this scratch_ext is not "scratch" in the traditional KHO sense at all.
The traditional KHO scratch is what is passed from kernel to kernel and is
guaranteed to contain zero preserved memory. This new memory is not passed
from kernel to kernel and can contain preserved memory at runtime. It's
essentially just memory that we identify as currently unpreserved and release
early to the system.

If we want to keep the naming aligned with the existing codebase for now:
MEMBLOCK_KHO_SCRATCH      -> original scratch
MEMBLOCK_KHO_UNPRESERVED  -> for the new memory (instead of SCRATCH_EXT)

Alternatively, if we do want to tackle the global rename of "scratch" later:
MEMBLOCK_KHO_BOOTSTRAP    -> for the original scratch
MEMBLOCK_KHO_UNPRESERVED  -> for this new dynamic memory

What do you think?

Pasha

> 
> >
> >> Signed-off-by: Pratyush Yadav (Google) <pratyush at kernel.org>
> >> ---
> >> 
> >> Notes:
> >>     Checkpatch complains about no space after MEMBLOCK_KHO_SCRATCH_EXT in
> >>     the declaration, but doing so makes it nicely align with all the other
> >>     numbers. Mike, if you'd like I can add some whitespace.
> >> 
> >>  include/linux/memblock.h | 10 ++++++++++
> >>  mm/memblock.c            | 41 ++++++++++++++++++++++++++++++++++------
> >>  2 files changed, 45 insertions(+), 6 deletions(-)
> 
> -- 
> Regards,
> Pratyush Yadav