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

[Usama Arif <usamaarif642@gmail.com>]

On 04/12/2025 14:04, Pasha Tatashin wrote:
> On Sun, Nov 30, 2025 at 3:52 AM Mike Rapoport <rppt@kernel.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@meta.com>
>>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>>> Reviewed-by: Pratyush Yadav <pratyush@kernel.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?

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?


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);
+       }
 }
 > 
>>> ---
>>>  mm/memblock.c | 14 +++++++++-----
>>>  1 file changed, 9 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/memblock.c b/mm/memblock.c
>>> index 8b13d5c28922a..913cf322eb89a 100644
>>> --- a/mm/memblock.c
>>> +++ b/mm/memblock.c
>>> @@ -20,8 +20,8 @@
>>>
>>>  #ifdef CONFIG_KEXEC_HANDOVER
>>>  #include <linux/libfdt.h>
>>> -#include <linux/kexec_handover.h>
>>>  #endif /* CONFIG_KEXEC_HANDOVER */
>>> +#include <linux/kexec_handover.h>
>>>
>>>  #include <asm/sections.h>
>>>  #include <linux/io.h>
>>> @@ -1126,8 +1126,10 @@ int __init_memblock memblock_reserved_mark_noinit(phys_addr_t base, phys_addr_t
>>>   */
>>>  __init int memblock_mark_kho_scratch(phys_addr_t base, phys_addr_t size)
>>>  {
>>> -     return memblock_setclr_flag(&memblock.memory, base, size, 1,
>>> -                                 MEMBLOCK_KHO_SCRATCH);
>>> +     if (is_kho_boot())
> 
> Looks like memblock_mark_kho_scratch() is called before is_kho_boot()
> is working:
> 
> [    0.000000] memblock_mark_kho_scratch: is_kho_boot: 0
> [    0.000000] memblock_mark_kho_scratch: is_kho_boot: 0
> [    0.000000] memblock_mark_kho_scratch: is_kho_boot: 0
> [    0.000000] KHO: kho_populate: is_kho_boot: 1
> [    0.000000] memblock_mark_kho_scratch: is_kho_boot: 1
> [    0.000000] memblock_clear_kho_scratch: is_kho_boot: 1
> ...
> 
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -1514,6 +1514,7 @@ void __init kho_populate(phys_addr_t fdt_phys,
> u64 fdt_len,
>         memblock_set_kho_scratch_only();
> 
>         kho_in.fdt_phys = fdt_phys;
> +       pr_err("%s: is_kho_boot: %d\n", __func__, is_kho_boot());
>         kho_in.scratch_phys = scratch_phys;
>         kho_scratch_cnt = scratch_cnt;
>         pr_info("found kexec handover data.\n");
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 87e7495a68c1..f55b5cdba5dd 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1126,6 +1126,7 @@ int __init_memblock
> memblock_reserved_mark_noinit(phys_addr_t base, phys_addr_t
>   */
>  __init int memblock_mark_kho_scratch(phys_addr_t base, phys_addr_t size)
>  {
> +       pr_err("%s: is_kho_boot: %d\n", __func__, is_kho_boot());
>         if (is_kho_boot())
>                 return memblock_setclr_flag(&memblock.memory, base, size, 1,
>                                             MEMBLOCK_KHO_SCRATCH);
> @@ -1142,6 +1143,7 @@ __init int memblock_mark_kho_scratch(phys_addr_t
> base, phys_addr_t size)
>   */
>  __init int memblock_clear_kho_scratch(phys_addr_t base, phys_addr_t size)
>  {
> +       pr_err("%s: is_kho_boot: %d\n", __func__, is_kho_boot());
>         if (is_kho_boot())
>                 return memblock_setclr_flag(&memblock.memory, base, size, 0,
>                                             MEMBLOCK_KHO_SCRATCH);
> 
>>> +             return memblock_setclr_flag(&memblock.memory, base, size, 1,
>>> +                                         MEMBLOCK_KHO_SCRATCH);
>>> +     return 0;
>>>  }
>>>
>>>  /**
>>> @@ -1140,8 +1142,10 @@ __init int memblock_mark_kho_scratch(phys_addr_t base, phys_addr_t size)
>>>   */
>>>  __init int memblock_clear_kho_scratch(phys_addr_t base, phys_addr_t size)
>>>  {
>>> -     return memblock_setclr_flag(&memblock.memory, base, size, 0,
>>> -                                 MEMBLOCK_KHO_SCRATCH);
>>> +     if (is_kho_boot())
>>> +             return memblock_setclr_flag(&memblock.memory, base, size, 0,
>>> +                                         MEMBLOCK_KHO_SCRATCH);
>>> +     return 0;
>>>  }
>>>
>>>  static bool should_skip_region(struct memblock_type *type,
>>> --
>>> 2.47.3
>>>
>>
>> --
>> Sincerely yours,
>> Mike.
>>