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Subject[PATCH v2] zswap: Update with same-value filled page feature
From: Srividya Desireddy <>
Date: Wed, 6 Dec 2017 16:29:50 +0530
Subject: [PATCH v2] zswap: Update with same-value filled page feature

Changes since v1:
Updated to clarify about zswap.same_filled_pages_enabled parameter.

Updated zswap document with details on same-value filled
pages identification feature.
The usage of zswap.same_filled_pages_enabled module parameter
is explained.

Signed-off-by: Srividya Desireddy <>
Documentation/vm/zswap.txt | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/Documentation/vm/zswap.txt b/Documentation/vm/zswap.txt
index 89fff7d..0b3a114 100644
--- a/Documentation/vm/zswap.txt
+++ b/Documentation/vm/zswap.txt
@@ -98,5 +98,25 @@ request is made for a page in an old zpool, it is uncompressed using its
original compressor. Once all pages are removed from an old zpool, the zpool
and its compressor are freed.

+Some of the pages in zswap are same-value filled pages (i.e. contents of the
+page have same value or repetitive pattern). These pages include zero-filled
+pages and they are handled differently. During store operation, a page is
+checked if it is a same-value filled page before compressing it. If true, the
+compressed length of the page is set to zero and the pattern or same-filled
+value is stored.
+Same-value filled pages identification feature is enabled by default and can be
+disabled at boot time by setting the "same_filled_pages_enabled" attribute to 0,
+e.g. zswap.same_filled_pages_enabled=0. It can also be enabled and disabled at
+runtime using the sysfs "same_filled_pages_enabled" 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.
A debugfs interface is provided for various statistic about pool size, number
-of pages stored, and various counters for the reasons pages are rejected.
+of pages stored, same-value filled pages and various counters for the reasons
+pages are rejected.
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