Re: [PATCH 10/12] kho: extended scratch

[Mike Rapoport <rppt@kernel.org>]

On Wed, Apr 29, 2026 at 03:39:12PM +0200, Pratyush Yadav wrote:
> From: "Pratyush Yadav (Google)" <pratyush@kernel.org>
>
> Methodology
> ===========
> 
> Introduce extended scratch areas. These areas are discovered at boot by
> walking the preserved memory radix tree and looking for free blocks of
> memory. They then marked as scratch to allow allocations from them. This
> makes KHO more resilient to memory pressure and allows supporting huge
> page preservation.
> 
> Since the preserved memory radix tree mixes both physical address and
> order into a single key, and does not track table pages, it is difficult
> to identify free areas from it directly. Walk the tree and digest it
> down into another radix tree. The latter tracks blocks of
> KHO_EXT_SHIFT (1 GiB as of now) granularity. Then walk the digested tree
> and mark the areas between the present keys as scratch.
>
> Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
> ---
>  include/linux/kexec_handover.h     |   1 +
>  kernel/liveupdate/kexec_handover.c | 148 +++++++++++++++++++++++++----
>  mm/mm_init.c                       |   1 +
>  3 files changed, 133 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 1a04e089f779..c2b843a5fb28 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -840,6 +857,120 @@ static void __init kho_reserve_scratch(void)
>  	kho_enable = false;
>  }
>  
> +#define KHO_EXT_SHIFT 30 /* 1 GiB */

arm64 does not necessarily use 1G gigantic pages and worse, it can have 2
gigantic hstates.

I think this should take into account what actual gigantic page sizes are
in use for the general case.

-- 
Sincerely yours,
Mike.