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

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 19/12/25 09:45, Ritesh Harjani (IBM) wrote:
> Sourabh Jain <sourabhjain@linux.ibm.com> writes:
>
>> 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.
>>
>> 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
>>
> And that every other architecture will have to duplicate this in their
> arch_hugetlb_valid_size() whenever they face the same problem.
>
> Instead like at other places, hugepages_supported() should also be
> checked in the following cmdlines setup functions.
>
>> 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.
>>
> Right. Thanks for taking care of it. I guess after this patch [1] moves
> hugetlbpage_init_defaultsize() to mmu_early_init_devtree(), it's good to
> bring back these checks in the respective cmdline setup functions which
> was removed as part of commit [2]
>
> [1]:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2354ad252b66695be02f4acd18e37bf6264f0464
>
> [2]: https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c2833a5bf75b3657c4dd20b3709c8c702754cb1f
>
> LGTM. Please feel free to add:
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Thank you for the review Ritesh.

- Sourabh Jain