mm: add docs for per-order mTHP split counters

This commit introduces documentation for mTHP split counters in
transhuge.rst.

[ioworker0@gmail.com: improve the doc as suggested by Ryan]
  Link: https://lkml.kernel.org/r/20240704012905.42971-3-ioworker0@gmail.com
[ioworker0@gmail.com: tweak Documentation/admin-guide/mm/transhuge.rst]
  Link: https://lkml.kernel.org/r/20240707013659.1151-1-ioworker0@gmail.com
Link: https://lkml.kernel.org/r/20240628130750.73097-3-ioworker0@gmail.comSigned-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Bang Li <libang.li@antgroup.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/transhuge.rst

@@ -369,10 +369,6 @@ also applies to the regions registered in khugepaged.
 Monitoring usage
 ================
 
-.. note::
-   Currently the below counters only record events relating to
-   PMD-sized THP. Events relating to other THP sizes are not included.
-
 The number of PMD-sized anonymous transparent huge pages currently used by the
 system is available by reading the AnonHugePages field in ``/proc/meminfo``.
 To identify what applications are using PMD-sized anonymous transparent huge
@@ -514,6 +510,21 @@ file_fallback_charge
 	falls back to using small pages even though the allocation was
 	successful.
 
+split
+	is incremented every time a huge page is successfully split into
+	smaller orders. This can happen for a variety of reasons but a
+	common reason is that a huge page is old and is being reclaimed.
+
+split_failed
+	is incremented if kernel fails to split huge
+	page. This can happen if the page was pinned by somebody.
+
+split_deferred
+        is incremented when a huge page is put onto split queue.
+        This happens when a huge page is partially unmapped and splitting
+        it would free up some memory. Pages on split queue are going to
+        be split under memory pressure, if splitting is possible.
+
 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