Re: [PATCH 1/2] kho: skip zero-sized lowmem and global bootmem areas

[Pratyush Yadav]

On Tue, Nov 03 2026, Mike Rapoport wrote:

> On Tue, Nov 03, 2026 at 03:14:36PM +0100, Pratyush Yadav wrote:
>> Hi Mike,
>> 
>> On Tue, Nov 03 2026, Mike Rapoport (Microsoft) wrote:
>> 
>> > The lowmem and the global bootmem areas can have zero size, for example
>> > when none or all of the memory is below ARCH_LOW_ADDRESS_LIMIT, or when
>> > either of those areas is explicitly set to zero with kho_bootmem=.
>> >
>> > For a zero-sized allocation memblock returns an address without reserving
>> > anything, and that empty area is recorded in kho_bootmem. If there is no
>> > memory below ARCH_LOW_ADDRESS_LIMIT, allocation of the empty lowmem area
>> > fails and KHO gets disabled.
>> >
>> > Extract a helper that checks for zero size and allocates memory from
>> > memblock, and use it to skip empty areas the same way as empty per-node
>> > areas are skipped.
>> >
>> > Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
>> > Reported-by: Chen Pei <cp0613@xxxxxxxxxxxxxxxxx>
>> > Closes: https://lore.kernel.org/all/20261026131851.9942-1-cp0613@xxxxxxxxxxxxxxxxx
>> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
>> 
>> Thanks for the patch! LGTM for the most part, one minor nitpick below.
>> 
>> > ---
>> >  kernel/liveupdate/kexec_handover.c | 61 ++++++++++++++++++--------------------
>> >  1 file changed, 29 insertions(+), 32 deletions(-)
>> >
>> > diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
>> > index 6f1f55fe31819..f268c57413a7e 100644
>> > --- a/kernel/liveupdate/kexec_handover.c
>> > +++ b/kernel/liveupdate/kexec_handover.c
>> > @@ -820,6 +820,27 @@ bool kho_bootmem_overlap(phys_addr_t phys, size_t size)
>> >  	return false;
>> >  }
>> >  
>> > +static int __init kho_reserve_bootmem_area(int *idx, phys_addr_t size,
>> > +					   phys_addr_t end, int nid)
>> > +{
>> > +	phys_addr_t addr;
>> > +	int i = *idx;
>> > +
>> > +	if (!size)
>> > +		return 0;
>> > +
>> > +	addr = memblock_alloc_range_nid(size, BOOTMEM_ALIGNMENT_BYTES, 0, end,
>> > +					nid, true);
>> > +	if (!addr)
>> > +		return -ENOMEM;
>> > +
>> > +	kho_bootmem[i].addr = addr;
>> > +	kho_bootmem[i].size = size;
>> > +	*idx = i + 1;
>> > +
>> > +	return 0;
>> 
>> I reckon it will be cleaner if you update kho_bootmem_cnt here directly.
>> So you directly do:
>> 
>> 	kho_bootmem[kho_bootmem_cnt].addr = addr;
>> 	kho_bootmem[kho_bootmem_cnt].size = size;
>> 	kho_bootmem_cnt++;
>> 
>> You won't have to pass around *idx and you won't need the final
>> kho_bootmem_cnt assignment.
>
> Ack.
>  
>> You might also want to have a kho_bootmem_max_cnt and do some bounds
>> checking here.
>
> Hmm, feels like over protecting. And if future patches will miss it AI will
> yell anyway :)

All right, fair enough.

-- 
Regards,
Pratyush Yadav