linux-kernel - Re: [PATCH v3 2/2] mm/memblock: only mark/clear KHO scratch memory when needed

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Message-ID: <aTHKaaeY7_HCswbg@kernel.org>
Date: Thu, 4 Dec 2025 19:52:41 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,
	Andrew Morton <akpm@...ux-foundation.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 v3 2/2] mm/memblock: only mark/clear KHO scratch memory
 when needed

Hi Usama,

On Thu, Dec 04, 2025 at 02:51:00PM +0000, Usama Arif wrote:
> > On Sun, Nov 30, 2025 at 3:52 AM Mike Rapoport <rppt@...nel.org> wrote:
> >>
> >> On Fri, Nov 28, 2025 at 05:29:34PM +0000, Usama Arif wrote:
> >>> The scratch memory for kexec handover is used to bootstrap the
> >>> kexec'ed kernel. Only the 1st 1MB is used as scratch, and its a
> >>> hack to get around limitations with KHO. It is only needed when
> >>> CONFIG_KEXEC_HANDOVER is enabled and only if it is a KHO boot
> >>> (both checked by is_kho_boot). Add check to prevent marking a KHO
> >>> scratch region unless needed.
> >>
> >> I'm going to rewrite the changelog and queue this for upstream:
> >>
> >> The scratch memory for kexec handover is used to bootstrap the kexec'ed
> >> kernel and it is only needed when it is a KHO boot, i.e. a kexec boot with
> >> handover data passed from the previous kernel.
> >>
> >> Currently x86 marks the first megabyte of memory as KHO scratch even for
> >> non-KHO boots if CONFIG_KEXEC_HANDOVER is enabled.
> >>
> >> Add check to prevent marking a KHO scratch regions unless they are actually
> >> needed.
> >>
> >>> Fixes: a2daf83e10378 ("x86/e820: temporarily enable KHO scratch for memory below 1M")
> >>> Reported-by: Vlad Poenaru <thevlad@...a.com>
> >>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> >>> Reviewed-by: Pratyush Yadav <pratyush@...nel.org>
> > 
> > This patch causes panic with my tests in linux-next.
> > 
> > [    0.000000] Kernel panic - not syncing: Cannot allocate 17280 bytes
> > for node 0 data
> > [    0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
> > 6.18.0-next-20251203 #2 PREEMPT(undef)
> > [    0.000000] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
> > BIOS 0.1 11/11/2019
> > [    0.000000] Call Trace:
> > [    0.000000]  <TASK>
> > [    0.000000]  ? dump_stack_lvl+0x4e/0x70
> > [    0.000000]  ? vpanic+0xcf/0x2b0
> > [    0.000000]  ? panic+0x66/0x66
> > [    0.000000]  ? alloc_node_data+0x32/0x90
> > [    0.000000]  ? numa_register_nodes+0x82/0x100
> > [    0.000000]  ? numa_init+0x36/0x120
> > [    0.000000]  ? setup_arch+0x667/0x7f0
> > [    0.000000]  ? start_kernel+0x58/0x640
> > [    0.000000]  ? x86_64_start_reservations+0x24/0x30
> > [    0.000000]  ? x86_64_start_kernel+0xc5/0xd0
> > [    0.000000]  ? common_startup_64+0x13e/0x148
> > [    0.000000]  </TASK>
> > [    0.000000] ---[ end Kernel panic - not syncing: Cannot allocate
> > 17280 bytes for node 0 data ]---
> > PANIC: early exception 0x0d IP 10:ffffffff89007a13 error 763 cr2
> > 0xffff991090a01000
> > 
> 
> Thanks for reporting this and sorry for the bug!
> 
> So the patch was designed to remove the memblock_mark_kho_scratch in e820__memblock_setup if not
> in KHO boot. But it broke memblock_mark_kho_scratch in kho_populate.
> Moving kho_in.fdt_phys = fdt_phys to before the memblock_mark_scratch
> should fix it. I dont have a setup where I can easily test KHO, but I think below
> should fix it?

This might, but this is too late for v6.19-rc1.
For now I'm dropping this series from memblock/for-next.
We can resume working on this after merge window closes.
 
> TBH using fdt_phys to check if the boot is KHO might be a bit hacky? Is it possible
> to have a better check for this?

Presence of KHO FDT is a clear indication that it is a KHO boot.
The issue is that during early boot ordering is hard and it's not always
clear in which order features and configuration are detected and used. 
 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 9dc51fab604f1..c331749e6452e 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -1483,6 +1483,7 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
>                 goto out;
>         }
>  
> +       kho_in.fdt_phys = fdt_phys;
>         /*
>          * We pass a safe contiguous blocks of memory to use for early boot
>          * purporses from the previous kernel so that we can resize the
> @@ -1513,7 +1514,6 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
>          */
>         memblock_set_kho_scratch_only();
>  
> -       kho_in.fdt_phys = fdt_phys;
>         kho_in.scratch_phys = scratch_phys;
>         kho_scratch_cnt = scratch_cnt;
>         pr_info("found kexec handover data.\n");
> @@ -1524,7 +1524,10 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
>         if (scratch)
>                 early_memunmap(scratch, scratch_len);
>         if (err)
> +       {
> +               kho_in.fdt_phys = 0;
>                 pr_warn("disabling KHO revival: %d\n", err);
> +       }
>  }

-- 
Sincerely yours,
Mike.