Re: [PATCH] kho: Skip zero-sized bootmem areas

[Pratyush Yadav <pratyush@kernel.org>]

On Mon, Oct 26 2026, Chen Pei wrote:

> The lowmem and the global bootmem area are reserved unconditionally,
> while the per-node loop below already skips nodes whose size works out
> to 0. Either can legitimately be 0: the global size is the scaled total
> minus the lowmem one, so it is 0 whenever all measured reservations fit
> below ARCH_LOW_ADDRESS_LIMIT.
>
> Reserving 0 bytes still succeeds, so an area covering nothing gets
> recorded with a valid address. That is not neutral:
> kho_bootmem_overlap() reports an overlap for any request straddling
> that address, and memblock_alloc_hugetlb() aligns a request to its page
> size, so a 1G hugetlb allocation retries a free candidate on account of
> an area that holds nothing.
>
> A machine whose memory all starts at or above ARCH_LOW_ADDRESS_LIMIT
> hits the other side of the same code: the lowmem size is 0, the range
> below the limit is empty, the allocation fails and KHO is disabled
> although no lowmem area was needed.
>
> Skip a zero-sized area instead, as the per-node loop does. The
> descriptor count stays consistent, as kho_populate() derives it from
> the segment length that kho_fill_kimage() serialises.
>
> Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
> Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>

Do you see zero sized lowmem or global scratch in practice? If yes, how?
What's your setup?

Note that I already fixed this for per-node areas since those do end up
with no allocations with movable_node set [0].

[0] https://lore.kernel.org/kexec/20261014131547.259138-1-pratyush@kernel.org/

[...]

-- 
Regards,
Pratyush Yadav