mm/hugetlb: add hugetlb_cma_only cmdline option

Add an option to force hugetlb gigantic pages to be allocated using CMA
only (if hugetlb_cma is enabled).  This avoids a fallback to allocation
from the rest of system memory if the CMA allocation fails.  This makes
the size of hugetlb_cma a hard upper boundary for gigantic hugetlb page
allocations.

This is useful because, with a large CMA area, the kernel's unmovable
allocations will have less room to work with and it is undesirable for new
hugetlb gigantic page allocations to be done from that remaining area.  It
will eat in to the space available for unmovable allocations, leading to
unwanted system behavior (OOMs because the kernel fails to do unmovable
allocations).

So, with this enabled, an administrator can force a hard upper bound for
runtime gigantic page allocations, and have more predictable system
behavior.

Link: https://lkml.kernel.org/r/20250228182928.2645936-26-fvdl@google.com


Signed-off-by: Frank van der Linden <fvdl@google.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>