mm/hugetlb: ignore hugepage kernel args if hugepages are unsupported

mm/hugetlb: ignore hugepage kernel args if hugepages are unsupported

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.

Note for backporting: This fix is a partial reversion of the commit
mentioned in the Fixes tag and is only valid once the change referenced by
the Depends-on tag is present.  When backporting this patch, the commit
mentioned in the Depends-on tag must be included first.

Link: https://lore.kernel.org/all/20250121150419.1342794-1-sourabhjain@linux.ibm.com/
Link: https://lore.kernel.org/all/20250128043358.163372-1-sourabhjain@linux.ibm.com/
Link: https://lkml.kernel.org/r/20251224115524.1272010-1-sourabhjain@linux.ibm.com
Fixes: c2833a5bf75b ("hugetlbfs: fix changes to command line processing")
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Depends-on: 2354ad252b66 ("powerpc/mm: Update default hugetlb size early")
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>