Re: [PATCH v6] mm/hugetlb: ignore hugepage kernel args if hugepages are unsupported

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

On 21/12/25 11:29, Ritesh Harjani (IBM) wrote:
> Hi Sourabh,
>
> 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, when the fadump kernel boots with the following kernel
>> arguments:
>> default_hugepagesz=1GB hugepagesz=1GB hugepages=200
>>
>> Before this patch, the kernel 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 state that HugeTLB support is disabled, gigantic
>> hugepages are still allocated. This causes the fadump kernel to run out
>> of memory during boot.
>>
>> After this patch is applied, the kernel prints the following logs for
>> the same command line:
>>
>> HugeTLB: hugepages unsupported, ignoring default_hugepagesz=1GB cmdline
>> HugeTLB: hugepages unsupported, ignoring hugepagesz=1GB cmdline
>> HugeTLB: hugepages unsupported, ignoring hugepages=200 cmdline
>> HugeTLB support is disabled!
>> hugetlbfs: disabling because there are no supported hugepage sizes
>>
>> To fix the issue, gigantic hugepage allocation should be guarded by
>> hugepages_supported().
>>
>> Previously, two approaches were proposed to bring gigantic hugepage
>> allocation under hugepages_supported():
>>
>> [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, skip processing hugepage kernel
>> arguments (default_hugepagesz, hugepagesz and hugepages) when
>> hugepages_supported() returns false.
>>
>> Link: https://lore.kernel.org/all/20250121150419.1342794-1-sourabhjain@linux.ibm.com/ [1]
>> Link: https://lore.kernel.org/all/20250128043358.163372-1-sourabhjain@linux.ibm.com/ [2]
>> Fixes: c2833a5bf75b ("hugetlbfs: fix changes to command line processing")
>
> I appreciate our proactiveness to respond quickly on mailing list, but I
> suggest we give enough time to folks before sending the next version
> please ;).

I agree that I posted the v6 too quickly. I will avoid that in future.

>
> Your email from last night [1] says that we will use this fixes tag but
> you haven't even given us 24hrs to respond to that email thread :). Now
> we've sent this v6, with Acked-by of David and Reviewed-by of mine,
> which seems like everything was agreed upon, but that isn't the case
> actually.

Yes, you are right. I should have waited until the discussion about the
Fixes tag was finished.

Thanks for pointing out things Ritesh.

- Sourabh Jain