Re: [PATCH v3] memblock: don't allocate new reserve_mem regions on a KHO boot

[Jose A. Perez de Azpillaga]

On 11/6/26 4:57 AM, Mukesh Pilaniya wrote:
> Hi Jose, Thanks for the patch!
> 
> On 06/11/26 3:01 am, Jose A. Perez de Azpillaga wrote:
>> On a KHO boot, a reserve_mem region that can't be revived is allocated
>> from KHO_NOPRSRV memory, which includes the bootmem the next kernel
>> boots out of. With CONFIG_KEXEC_HANDOVER_DEBUG, preserving the region
>> then fails:
>>
>>   WARNING: kernel/liveupdate/kexec_handover.c:1245 at kho_preserve_pages+0x168/0x180
>>   failed to prepare memblock FDT for KHO: -22
>>
>> Without it, the next kernel puts its memmap on top of the region and
>> the ramoops records in it are lost.
>>
>> There is no use case for adding regions across a live update, so fail
>> instead of allocating, and drop the TODO.
>>
>> Add an is_kho_boot() stub so the memblock tests still build.
>>
>> Fixes: f99230780211 ("memblock: add KHO support for reserve_mem")
>> Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
>> Signed-off-by: Jose A. Perez de Azpillaga <azpijr@xxxxxxxxx>
>> ---
>> Changes in v3:
>> - add an is_kho_boot() stub to the memblock tests (Mike).
>> - add Pasha's Reviewed-by.
>>
>> Changes in v2:
>> - fail on a KHO boot instead of allocating outside bootmem, as agreed
>>   with Pratyush [1]. this drops v1's patch 1.
>> - not an RFC anymore.
>> - based on v7.4-rc1.
>>
>> v2: https://lore.kernel.org/all/20261104204235.9066-1-azpijr@xxxxxxxxx/
>> v1: https://lore.kernel.org/all/20261021211058.83483-3-azpijr@xxxxxxxxx/
>>
>> [1] https://lore.kernel.org/all/2vxzfqxglrg6.fsf@xxxxxxxxxx/
>>
>>  mm/memblock.c                     | 11 ++++++++++-
>>  tools/testing/memblock/internal.h |  5 +++++
>>  2 files changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index a450092a80fb..e325e0542c9a 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -2827,7 +2827,16 @@ static int __init reserve_mem(char *p)
>>  	if (reserve_mem_kho_revive(name, size, align))
>>  		return 1;
>>  
>> -	/* TODO: Allocation must be outside of KHO_NOPRSRV region */
>> +	/*
>> +	 * On a KHO boot memblock only allocates from KHO_NOPRSRV memory here,
>> +	 * which includes the bootmem the next kernel boots out of. Only regions
>> +	 * handed over by the previous kernel are supported.
>> +	 */
>> +	if (is_kho_boot()) {
>> +		pr_err("reserve_mem: can't allocate \"%s\" on a KHO boot\n", name);
>> +		return -EOPNOTSUPP;
>> +	}
> I think this over-restricts when KHO is explicitly disabled. kho= is an
> early_param parsed before reserve_mem=, but kho_populate records the
> incoming FDT without checking kho_enable. So a KHO payload booted with
> kho=off ends up with is_kho_boot() == true and kho_is_enabled() ==false.
> 
> In that config nothing can be handed over again; reserve_mem_init
> returns early on !kho_is_enabled(), and kho_fill_kimage bails on
> !kho_enable. Allocating the region from KHO_NOPRSRV is therefore
> harmless, and the documented best-effort reserve_mem= behaviour is
> preserved.
> That said, a new reserve_mem= line is rejected and its ramoops/ftrace
> boot-instance consumers lose their region.
> 
> So, Would
> 
> 	if (kho_is_enabled() && is_kho_boot())
> 
> be better here? If so, the memblock tests will also need a
> kho_is_enabled() stub, since
> tools/testing/memblock/linux/kexec_handover.h is empty and reserve_mem()
> sits outside the CONFIG_KEXEC_HANDOVER ifdef.

good catch, will do it in v4 :)

-- 
cheers,
jose a. p-a