zswap: update docs for runtime-changeable attributes
Change the Documentation/vm/zswap.txt doc to indicate that the "zpool" and
"compressor" params are now changeable at runtime.
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/vm/zswap.txt b/Documentation/vm/zswap.txt
index 8458c08..89fff7d 100644
--- a/Documentation/vm/zswap.txt
+++ b/Documentation/vm/zswap.txt
@@ -32,7 +32,7 @@
An example command to enable zswap at runtime, assuming sysfs is mounted
at /sys, is:
-echo 1 > /sys/modules/zswap/parameters/enabled
+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
@@ -49,14 +49,26 @@
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.
@@ -74,9 +86,17 @@
* 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.