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

[Mike Rapoport]

On Tue, Nov 03, 2026 at 02:48:36PM -0500, Pasha Tatashin wrote:
> On 11-03 13:49, 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>
> > ---
> >  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;
> > +}
> > +
> >  /**
> >   * kho_reserve_bootmem - Reserve a contiguous chunk of memory for kexec
> >   *
> > @@ -832,7 +853,7 @@ bool kho_bootmem_overlap(phys_addr_t phys, size_t size)
> >  static void __init kho_reserve_bootmem(void)
> >  {
> >  	int nid, i = 0, max_nr_bootmem;
> > -	phys_addr_t addr, size;
> > +	phys_addr_t size;
> >  
> >  	if (!kho_enable)
> >  		return;
> > @@ -852,56 +873,32 @@ static void __init kho_reserve_bootmem(void)
> >  	 * reserve bootmem area in low memory for lowmem allocations in the
> >  	 * next kernel
> >  	 */
> > -	size = bootmem_size_lowmem;
> > -	addr = memblock_phys_alloc_range(size, BOOTMEM_ALIGNMENT_BYTES, 0,
> > -					 ARCH_LOW_ADDRESS_LIMIT);
> > -	if (!addr) {
> > +	if (kho_reserve_bootmem_area(&i, bootmem_size_lowmem,
> > +				     ARCH_LOW_ADDRESS_LIMIT, NUMA_NO_NODE)) {
> >  		pr_err("Failed to reserve lowmem bootmem\n");
> 
> You are adding a helper that returns an error code and then ignoring it 
> at all call sites. Even though it can only return -ENOMEM right now, 
> dropping the return value is not good practice.

I don't think error code is important here.
Even if some theoretical future update will change
kho_reserve_bootmem_area() to return anything but -ENOMEM I don't see why
the error code should be printed at all.

And anyway, that would be the part of that change.

-- 
Sincerely yours,
Mike.