These changes are the raw update to linux-4.4.6-rt14. Kernel sources

are taken from kernel.org, and rt patch from the rt wiki download page.

During the rebasing, the following patch collided:

Force tick interrupt and get rid of softirq magic(I70131fb85).

Collisions have been removed because its logic was found on the
source already.

Change-Id: I7f57a4081d9deaa0d9ccfc41a6c8daccdee3b769
Signed-off-by: José Pekkarinen <jose.pekkarinen@nokia.com>

1 files changed, 43 insertions, 9 deletions

diff --git a/kernel/Documentation/vm/zswap.txt b/kernel/Documentation/vm/zswap.txt
index 00c3d31e7..89fff7d61 100644
--- a/kernel/Documentation/vm/zswap.txt
+++ b/kernel/Documentation/vm/zswap.txt
@@ -26,8 +26,22 @@ Zswap evicts pages from compressed cache on an LRU basis to the backing swap
 device when the compressed pool reaches its size limit.  This requirement had
 been identified in prior community discussions.
 
-To enabled zswap, the "enabled" attribute must be set to 1 at boot time.  e.g.
-zswap.enabled=1
+Zswap is disabled by default but can be enabled at boot time by setting
+the "enabled" attribute to 1 at boot time. ie: zswap.enabled=1.  Zswap
+can also be enabled and disabled at runtime using the sysfs interface.
+An example command to enable zswap at runtime, assuming sysfs is mounted
+at /sys, is:
+
+echo 1 > /sys/module/zswap/parameters/enabled
+
+When zswap is disabled at runtime it will stop storing pages that are
+being swapped out.  However, it will _not_ immediately write out or fault
+back into memory all of the pages stored in the compressed pool.  The
+pages stored in zswap will remain in the compressed pool until they are
+either invalidated or faulted back into memory.  In order to force all
+pages out of the compressed pool, a swapoff on the swap device(s) will
+fault back into memory all swapped out pages, including those in the
+compressed pool.
 
 Design:
 
@@ -35,14 +49,26 @@ Zswap receives pages for compression through the Frontswap API and is able to
 evict pages from its own compressed pool on an LRU basis and write them back to
 the backing swap device in the case that the compressed pool is full.
 
-Zswap makes use of zbud for the managing the compressed memory pool.  Each
-allocation in zbud is not directly accessible by address.  Rather, a handle is
+Zswap makes use of zpool for the managing the compressed memory pool.  Each
+allocation in zpool is not directly accessible by address.  Rather, a handle is
 returned by the allocation routine and that handle must be mapped before being
 accessed.  The compressed memory pool grows on demand and shrinks as compressed
-pages are freed.  The pool is not preallocated.
+pages are freed.  The pool is not preallocated.  By default, a zpool of type
+zbud is created, but it can be selected at boot time by setting the "zpool"
+attribute, e.g. zswap.zpool=zbud.  It can also be changed at runtime using the
+sysfs "zpool" attribute, e.g.
+
+echo zbud > /sys/module/zswap/parameters/zpool
+
+The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
+means the compression ratio will always be 2:1 or worse (because of half-full
+zbud pages).  The zsmalloc type zpool has a more complex compressed page
+storage method, and it can achieve greater storage densities.  However,
+zsmalloc does not implement compressed page eviction, so once zswap fills it
+cannot evict the oldest page, it can only reject new pages.
 
 When a swap page is passed from frontswap to zswap, zswap maintains a mapping
-of the swap entry, a combination of the swap type and swap offset, to the zbud
+of the swap entry, a combination of the swap type and swap offset, to the zpool
 handle that references that compressed swap page.  This mapping is achieved
 with a red-black tree per swap type.  The swap offset is the search key for the
 tree nodes.
@@ -60,9 +86,17 @@ controlled policy:
 * max_pool_percent - The maximum percentage of memory that the compressed
     pool can occupy.
 
-Zswap allows the compressor to be selected at kernel boot time by setting the
-“compressor” attribute.  The default compressor is lzo.  e.g.
-zswap.compressor=deflate
+The default compressor is lzo, but it can be selected at boot time by setting
+the “compressor” attribute, e.g. zswap.compressor=lzo.  It can also be changed
+at runtime using the sysfs "compressor" attribute, e.g.
+
+echo lzo > /sys/module/zswap/parameters/compressor
+
+When the zpool and/or compressor parameter is changed at runtime, any existing
+compressed pages are not modified; they are left in their own zpool.  When a
+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.
 
 A debugfs interface is provided for various statistic about pool size, number
 of pages stored, and various counters for the reasons pages are rejected.