Re: [PATCH v5] mm/hugetlb: ignore hugepage kernel args if hugepages are unsupported

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/18/25 12:41, Sourabh Jain wrote:
> Skip processing hugepage kernel arguments (hugepagesz, hugepages, and
> default_hugepagesz) when hugepages are not supported by the
> architecture.
> 
> Some architectures may need to disable hugepages based on conditions
> discovered during kernel boot. The hugepages_supported() helper allows
> architecture code to advertise whether hugepages are supported.
> 
> Currently, normal hugepage allocation is guarded by
> hugepages_supported(), but gigantic hugepages are allocated regardless
> of this check. This causes problems on powerpc for fadump (firmware-
> assisted dump).
> 
> In the fadump (firmware-assisted dump) scenario, a production kernel
> crash causes the system to boot into a special kernel whose sole
> purpose is to collect the memory dump and reboot. Features such as
> hugepages are not required in this environment and should be
> disabled.
> 
> For example, fadump kernel booting with the kernel arguments
> default_hugepagesz=1GB hugepagesz=1GB hugepages=200 prints the
> following logs:
> 
> HugeTLB: allocating 200 of page size 1.00 GiB failed.  Only allocated 58 hugepages.
> HugeTLB support is disabled!
> HugeTLB: huge pages not supported, ignoring associated command-line parameters
> hugetlbfs: disabling because there are no supported hugepage sizes
> 
> Even though the logs say that hugetlb support is disabled, gigantic
> hugepages are still getting allocated, which causes the fadump kernel
> to run out of memory during boot.

Yeah, that's suboptimal.

> 
> To fix this, the gigantic hugepage allocation should come under
> hugepages_supported().
> 
> To bring gigantic hugepage allocation under hugepages_supported(), two
> approaches were previously proposed:
> [1] Check hugepages_supported() in the generic code before allocating
> gigantic hugepages.
> [2] Make arch_hugetlb_valid_size() return false for all hugetlb sizes.
> 
> Approach [2] has two minor issues:
> 1. It prints misleading logs about invalid hugepage sizes
> 2. The kernel still processes hugepage kernel arguments unnecessarily
> 
> To control gigantic hugepage allocation, it is proposed to skip
> processing the hugepage kernel arguments (hugepagesz, hugepages, and
> default_hugepagesz) when hugepages_support() returns false.

You could briefly mention the new output here, so one has a before-after 
comparison.

Curious, should we at least add a Fixes: tag? Allocating memory when 
it's completely unusable sounds wrong.

[...]

> +	if (!hugepages_supported()) {
> +		pr_warn("HugeTLB: hugepages unsupported, ignoring default_hugepagesz=%s cmdline\n",
> +			s);
> +		return 0;
> +	}
> +
>   	parsed_valid_hugepagesz = false;
>   	if (parsed_default_hugepagesz) {
>   		pr_err("HugeTLB: default_hugepagesz previously specified, ignoring %s\n", s);


LGTM!

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>

-- 
Cheers

David