mm: add docs for per-order mTHP split counters

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

[[email protected]: improve the doc as suggested by Ryan]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: tweak Documentation/admin-guide/mm/transhuge.rst]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mingzhe Yang <[email protected]>
Signed-off-by: Lance Yang <[email protected]>
Reviewed-by: Barry Song <[email protected]>
Reviewed-by: Ryan Roberts <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Bang Li <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 15 insertions, 4 deletions

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 1f72b00af5d3..a1bc9b24e29a 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/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