[RFC PATCH v2 9/9] docs: mm: update THP swapin counter descriptions

[fujunjie]

The THP swapin counter descriptions still describe large swapin as
coming only from non-zswap swap devices. Update them now that
zswap-backed large folio swapin can also increment swpin.

Also describe policy and backend rejection as swpin_fallback cases,
since speculative zswap large swapin can intentionally fall back before
doing large IO.

Signed-off-by: fujunjie <fujunjie1@xxxxxx>
---
 Documentation/admin-guide/mm/transhuge.rst | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 23f8d13c2629..59b7a0d09243 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -667,13 +667,14 @@ zswpout
 	piece without splitting.
 
 swpin
-	is incremented every time a huge page is swapped in from a non-zswap
-	swap device in one piece.
+	is incremented every time a huge page is swapped in from swap or
+	zswap in one piece.
 
 swpin_fallback
-	is incremented if swapin fails to allocate or charge a huge page
-	and instead falls back to using huge pages with lower orders or
-	small pages.
+	is incremented if swapin cannot use a huge page and instead falls
+	back to using huge pages with lower orders or small pages. This can
+	happen because allocation or charging fails, or because policy or
+	backend state rejects a speculative large swapin.
 
 swpin_fallback_charge
 	is incremented if swapin fails to charge a huge page and instead
-- 
2.34.1