linux-kernel - Re: [PATCH v2 2/2] mm/memblock: only mark/clear KHO scratch memory when needed

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Message-ID: <56673a9c-a4c9-4962-baec-2d4483af3cfa@gmail.com>
Date: Thu, 27 Nov 2025 21:04:58 +0000
From: Usama Arif <usamaarif642@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: rppt@...nel.org, kas@...nel.org, changyuanl@...gle.com, graf@...zon.com,
 leitao@...ian.org, thevlad@...a.com, pratyush@...nel.org,
 dave.hansen@...ux.intel.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v2 2/2] mm/memblock: only mark/clear KHO scratch memory
 when needed

On 27/11/2025 20:55, Andrew Morton wrote:
> On Thu, 27 Nov 2025 20:33:20 +0000 Usama Arif <usamaarif642@...il.com> wrote:
> 
>> The scratch memory for kexec handover is used to bootstrap the
>> kexec'ed kernel. It is only needed when CONFIG_KEXEC_HANDOVER
>> is enabled and only if it is a KHO boot. Add checks to prevent
>> marking a KHO scratch region unless needed.
> 
> What effect does this change have?  Lessened memory consumption,
> presumably.  Of what magnitude and for what time period?

For some context, this came out of https://lore.kernel.org/all/ba690e06-c2a1-4d2e-9428-9ca2ea9f2b86@gmail.com/
(I should have probably added that in the commit message..)
We are experiencing several warnings a day in meta fleet due to a warning introduced
in that patch. We dont have CONFIG_KEXEC_HANDOVER enabled in the fleet. The IMA memory
seems to conincide with the 1st MB, but as Mike pointed out they are different arrays
so this scratch memory is likely not a cause of the warnings. But it is not useful (and
was a bit confusing) seeing KHO scratch memory being marked even when KHO is disabled.

The imapct is as you said, but its only marked for a very short period of time.
I think a better reason for this patch is just to not mark the memory at all when KHO
is disabled (or not in use) for clarity.