mm: add docs for per-order mTHP counters and transhuge_page ABI

This patch includes documentation for mTHP counters and an ABI file for
sys-kernel-mm-transparent-hugepage, which appears to have been missing for
some time.

[v-songbaohua@oppo.com: fix the name and unexpected indentation]
  Link: https://lkml.kernel.org/r/20240415054538.17071-1-21cnbao@gmail.com
Link: https://lkml.kernel.org/r/20240412114858.407208-4-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/ABI/testing/sysfs-kernel-mm-transparent-hugepage

@@ -0,0 +1,18 @@
+What:		/sys/kernel/mm/transparent_hugepage/
+Date:		April 2024
+Contact:	Linux memory management mailing list <linux-mm@kvack.org>
+Description:
+		/sys/kernel/mm/transparent_hugepage/ contains a number of files and
+		subdirectories,
+
+			- defrag
+			- enabled
+			- hpage_pmd_size
+			- khugepaged
+			- shmem_enabled
+			- use_zero_page
+			- subdirectories of the form hugepages-<size>kB, where <size>
+			  is the page size of the hugepages supported by the kernel/CPU
+			  combination.
+
+		See Documentation/admin-guide/mm/transhuge.rst for details.

Documentation/admin-guide/mm/transhuge.rst

@@ -447,6 +447,34 @@ thp_swpout_fallback
 	Usually because failed to allocate some continuous swap space
 	for the huge page.
 
+In /sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats, There are
+also individual counters for each huge page size, which can be utilized to
+monitor the system's effectiveness in providing huge pages for usage. Each
+counter has its own corresponding file.
+
+anon_fault_alloc
+	is incremented every time a huge page is successfully
+	allocated and charged to handle a page fault.
+
+anon_fault_fallback
+	is incremented if a page fault fails to allocate or charge
+	a huge page and instead falls back to using huge pages with
+	lower orders or small pages.
+
+anon_fault_fallback_charge
+	is incremented if a page fault fails to charge a huge page and
+	instead falls back to using huge pages with lower orders or
+	small pages even though the allocation was successful.
+
+anon_swpout
+	is incremented every time a huge page is swapped out in one
+	piece without splitting.
+
+anon_swpout_fallback
+	is incremented if a huge page has to be split before swapout.
+	Usually because failed to allocate some continuous swap space
+	for the huge page.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in ``/proc/vmstat`` to help