mm, THP, doc: Add document for thp_swpout/thp_swpout_fallback

Add document for newly added thp_swpout, thp_swpout_fallback fields in
/proc/vmstat.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/vm/transhuge.rst b/Documentation/vm/transhuge.rst
index 569d182cc973..2c6867fca6ff 100644
--- a/Documentation/vm/transhuge.rst
+++ b/Documentation/vm/transhuge.rst
@@ -355,6 +355,15 @@ thp_zero_page_alloc_failed
         is incremented if kernel fails to allocate
         huge zero page and falls back to using small pages.
 
+thp_swpout
+        is incremented every time a huge page is swapout in one
+        piece without splitting.
+
+thp_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