Introduce the logic to allow THP to be configured (through the new sysfs
interface we just added) to allocate large folios to back anonymous
memory, which are larger than the base page size but smaller than
PMD-size.  We call this new THP extension "multi-size THP" (mTHP).

mTHP continues to be PTE-mapped, but in many cases can still provide
similar benefits to traditional PMD-sized THP: Page faults are
significantly reduced (by a factor of e.g.  4, 8, 16, etc.  depending on
the configured order), but latency spikes are much less prominent because
the size of each page isn't as huge as the PMD-sized variant and there is
less memory to clear in each page fault.  The number of per-page
operations (e.g.  ref counting, rmap management, lru list management) are
also significantly reduced since those ops now become per-folio.

Some architectures also employ TLB compression mechanisms to squeeze more
entries in when a set of PTEs are virtually and physically contiguous and
approporiately aligned.  In this case, TLB misses will occur less often.

The new behaviour is disabled by default, but can be enabled at runtime by
writing to /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled (see
documentation in previous commit).  The long term aim is to change the
default to include suitable lower orders, but there are some risks around
internal fragmentation that need to be better understood first.

[ryan.roberts@arm.com: resolve some multi-size THP review nits]
  Link: https://lkml.kernel.org/r/20231214160251.3574571-1-ryan.roberts@arm.com
Link: https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Rientjes <rientjes@google.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Itaru Kitayama <itaru.kitayama@gmail.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yin Fengwei <fengwei.yin@intel.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>