linux-kernel - Re: [PATCH] liveupdate: kho: Enable KHO by default

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Message-ID: <CA+CK2bD+ZseJ_6GUMV7d=8xpUcWW6jzJBdJRy7kQmez_2sMYUQ@mail.gmail.com>
Date: Fri, 14 Nov 2025 09:13:01 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: akpm@...ux-foundation.org, graf@...zon.com, pratyush@...nel.org, 
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH] liveupdate: kho: Enable KHO by default

On Fri, Nov 14, 2025 at 2:30 AM Mike Rapoport <rppt@...nel.org> wrote:
>
> On Mon, Nov 10, 2025 at 01:07:15PM -0500, Pasha Tatashin wrote:
> > Upcoming LUO requires KHO for its operations, the requirement to place
> > both KHO=on and liveupdate=on becomes redundant. Set KHO to be enabled
> > by default.
>
> I though more about this and it seems too much of a change. kho=1 enables
> scratch areas and that significantly changes how free pages are distributed
> in the free lists.
>
> Let's go with a Kconfig option we discussed of-list:
> (this is on top of the current mmotm/mm-nonmm-unstable)

I will include this into the KHO simplification series

>
> From 823299d80aa4f7c16ef6cfd798a19e1dfe1a91ab Mon Sep 17 00:00:00 2001
> From: Pasha Tatashin <pasha.tatashin@...een.com>
> Date: Fri, 14 Nov 2025 09:27:47 +0200
> Subject: [PATCH] kho: Allow KHO to be enabled by default
>
> Upcoming LUO requires KHO for its operations, the requirement to place
> both KHO=on and liveupdate=on becomes reduntant. Let's allow KHO to be
> enabled by default, and CONFIG_LIVEUPDATE can select this CONFIG.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> ---
>  kernel/liveupdate/Kconfig          | 8 ++++++++
>  kernel/liveupdate/kexec_handover.c | 2 +-
>  2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/liveupdate/Kconfig b/kernel/liveupdate/Kconfig
> index d7344d347f69..25c9a4d7781f 100644
> --- a/kernel/liveupdate/Kconfig
> +++ b/kernel/liveupdate/Kconfig
> @@ -63,4 +63,12 @@ config KEXEC_HANDOVER_DEBUGFS
>           Also, enables inspecting the KHO fdt trees with the debugfs binary
>           blobs.
>
> +config KEXEC_HANDOVER_ENABLE_DEFAULT
> +       bool "Enable kexec handover by default"
> +       depends on KEXEC_HANDOVER
> +       help
> +         Enable the kexec handover by default. It is equivalent of passing
> +         kho=on via kernel parameter, and can be overwritten to off via
> +         kho=off.
> +
>  endmenu
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 568cd9fe9aca..23a3df297bb3 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -51,7 +51,7 @@ union kho_page_info {
>
>  static_assert(sizeof(union kho_page_info) == sizeof(((struct page *)0)->private));
>
> -static bool kho_enable __ro_after_init = true;
> +static bool kho_enable __ro_after_init = IS_ENABLED(CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT);
>
>  bool kho_is_enabled(void)
>  {
> --
> 2.50.1
>
>
>
> --
> Sincerely yours,
> Mike.