[PATCH] mm/zswap.c: allow handling just same-value filled pages

["Maciej S. Szmigiero" <mail@maciej.szmigiero.name>]

From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>

Zswap has an ability to efficiently store same-value filled pages, which
can be turned on and off using the "same_filled_pages_enabled" parameter.

However, there is currently no way to enable just this (lightweight)
functionality, while not making use of the whole compressed page storage
machinery.

Add a "non_same_filled_pages_enabled" parameter which allows disabling
handling of pages that aren't same-value filled.
This way zswap can be run in such lightweight same-value filled pages only
mode.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
---
 Documentation/admin-guide/mm/zswap.rst | 22 +++++++++++++++++++---
 mm/zswap.c                             | 15 ++++++++++++++-
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
index 8edb8d578caf..6e6f7b0d6562 100644
--- a/Documentation/admin-guide/mm/zswap.rst
+++ b/Documentation/admin-guide/mm/zswap.rst
@@ -130,9 +130,25 @@ attribute, e.g.::
 	echo 1 > /sys/module/zswap/parameters/same_filled_pages_enabled
 
 When zswap same-filled page identification is disabled at runtime, it will stop
-checking for the same-value filled pages during store operation. However, the
-existing pages which are marked as same-value filled pages remain stored
-unchanged in zswap until they are either loaded or invalidated.
+checking for the same-value filled pages during store operation.
+In other words, every page will be then considered non-same-value filled.
+However, the existing pages which are marked as same-value filled pages remain
+stored unchanged in zswap until they are either loaded or invalidated.
+
+In some circumstances it might be advantageous to make use of just the zswap
+ability to efficiently store same-filled pages without enabling the whole
+compressed page storage.
+In this case the handling of non-same-value pages by zswap (enabled by default)
+can be disabled by setting the ``non_same_filled_pages_enabled`` attribute
+to 0, e.g. ``zswap.non_same_filled_pages_enabled=0``.
+It can also be enabled and disabled at runtime using the sysfs
+``non_same_filled_pages_enabled`` attribute, e.g.::
+
+	echo 1 > /sys/module/zswap/parameters/non_same_filled_pages_enabled
+
+Disabling both ``zswap.same_filled_pages_enabled`` and
+``zswap.non_same_filled_pages_enabled`` effectively disables accepting any new
+pages by zswap.
 
 To prevent zswap from shrinking pool when zswap is full and there's a high
 pressure on swap (this will result in flipping pages in and out zswap pool
diff --git a/mm/zswap.c b/mm/zswap.c
index 7944e3e57e78..7584ab85c7c7 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -120,11 +120,19 @@ static unsigned int zswap_accept_thr_percent = 90; /* of max pool size */
 module_param_named(accept_threshold_percent, zswap_accept_thr_percent,
 		   uint, 0644);
 
-/* Enable/disable handling same-value filled pages (enabled by default) */
+/*
+ * Enable/disable handling same-value filled pages (enabled by default).
+ * If disabled every page is considered non-same-value filled.
+ */
 static bool zswap_same_filled_pages_enabled = true;
 module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
 		   bool, 0644);
 
+/* Enable/disable handling non-same-value filled pages (enabled by default) */
+static bool zswap_non_same_filled_pages_enabled = true;
+module_param_named(non_same_filled_pages_enabled, zswap_non_same_filled_pages_enabled,
+		   bool, 0644);
+
 /*********************************
 * data structures
 **********************************/
@@ -1147,6 +1155,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 		kunmap_atomic(src);
 	}
 
+	if (!zswap_non_same_filled_pages_enabled) {
+		ret = -EINVAL;
+		goto freepage;
+	}
+
 	/* if entry is successfully added, it keeps the reference */
 	entry->pool = zswap_pool_current_get();
 	if (!entry->pool) {