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

[Mike Rapoport <rppt@kernel.org>]

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@linux.alibaba.com>
> > Closes: https://lore.kernel.org/all/20261026131851.9942-1-cp0613@linux.alibaba.com
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> 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 :)
 
> -- 
> Regards,
> Pratyush Yadav

-- 
Sincerely yours,
Mike.