Hello Gerald,

On 22/01/25 19:36, Gerald Schaefer wrote:

On Tue, 21 Jan 2025 20:34:19 +0530
Sourabh Jain [off-list ref] wrote:

quoted

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 <redacted>
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 <redacted>
---

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