linux-kernel - Re: [PATCH] mm/hugetlb: bring gigantic page allocation under hugepages

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Message-ID: <f6576f1c-bba3-44cc-bcb4-95318d2ece5c@linux.ibm.com>
Date: Thu, 23 Jan 2025 09:00:10 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc: akpm@...ux-foundation.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: bring gigantic page allocation under
 hugepages_supported()

Hello Gerald,

On 22/01/25 19:36, Gerald Schaefer wrote:
> On Tue, 21 Jan 2025 20:34:19 +0530
> Sourabh Jain <sourabhjain@...ux.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@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Borislav Petkov <bp@...en8.de>
>> Cc: Heiko Carstens <hca@...ux.ibm.com>
>> Cc: Vasily Gorbik <gor@...ux.ibm.com>
>> Cc: Muchun Song <muchun.song@...ux.dev>
>> Cc: Madhavan Srinivasan <maddy@...ux.ibm.com>
>> Cc: Michael Ellerman <mpe@...erman.id.au>
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> Cc: linuxppc-dev@...ts.ozlabs.org
>> Signed-off-by: Sourabh Jain <sourabhjain@...ux.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.

- Sourabh Jain