Re: [PATCH] mm/hugetlb: bring gigantic page allocation under hugepages_supported()

[Hari Bathini <hbathini@linux.ibm.com>]

On 23/01/25 9:00 am, Sourabh Jain wrote:
> Hello Gerald,
> 
> On 22/01/25 19:36, Gerald Schaefer wrote:
>> On Tue, 21 Jan 2025 20:34:19 +0530
>> Sourabh Jain <sourabhjain@linux.ibm.com> wrote:
>>
>>> Despite having kernel arguments to enable gigantic hugepages, this
>>> provides a way for the architecture to disable gigantic hugepages on the
>>> fly, similar to what we do for hugepages.
>>>
>>> Components like fadump (PowerPC-specific) need this functionality to
>>> disable gigantic hugepages when the kernel is booted solely to collect
>>> the kernel core dump.
>>>
>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>> Cc: Ingo Molnar <mingo@redhat.com>
>>> Cc: Borislav Petkov <bp@alien8.de>
>>> Cc: Heiko Carstens <hca@linux.ibm.com>
>>> Cc: Vasily Gorbik <gor@linux.ibm.com>
>>> Cc: Muchun Song <muchun.song@linux.dev>
>>> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
>>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>>> Cc: linux-mm@kvack.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Cc: linuxppc-dev@lists.ozlabs.org
>>> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
>>> ---
>>>
>>> To evaluate the impact of this change on architectures other than
>>> PowerPC, I did the following analysis:
>>>
>>> For architectures where hugepages_supported() is not redefined, it
>>> depends on HPAGE_SHIFT, which is found to be a constant. It is mostly
>>> initialized to PMD_SHIFT.
>>>
>>> Architecture : HPAGE_SHIFT initialized with
>>>
>>> ARC: PMD_SHIFT (constant)
>>> ARM: PMD_SHIFT (constant)
>>> ARM64: PMD_SHIFT (constant)
>>> Hexagon: 22 (constant)
>>> LoongArch: (PAGE_SHIFT + PAGE_SHIFT - 3) (appears to be constant)
>>> MIPS: (PAGE_SHIFT + PAGE_SHIFT - 3) (appears to be constant)
>>> PARISC: PMD_SHIFT (appears to be constant)
>>> RISC-V: PMD_SHIFT (constant)
>>> SH: 16 | 18 | 20 | 22 | 26 (constant)
>>> SPARC: 23 (constant)
>>>
>>> So seems like this change shouldn't have any impact on above
>>> architectures.
>>>
>>> On the S390 and X86 architectures, hugepages_supported() is redefined,
>>> and I am uncertain at what point it is safe to call
>>> hugepages_supported().
>> For s390, hugepages_supported() checks EDAT1 machine flag, which is
>> initialized long before any initcalls. So it is safe to be called
>> here.
> Thanks for the info.
>>
>> My common code hugetlb skills got a little rusty, but shouldn't
>> arch_hugetlb_valid_size() already prevent getting here for gigantic
>> hugepages, in case they are not supported? And could you not use
>> that for your purpose?
> 
> Yes, handling this in arch_hugetlb_valid_size is even better. That way,
> we can avoid initializing data structures to hold hstate, which is not
> required anyway.
> 
> Thanks for the review and suggestion. I will handle this in the
> architecture-specific code.

Yeah, adding a check for hugetlb_disabled in arch_hugetlb_valid_size()
should take care of things?

- Hari