[PATCHv10 4/4] zswap: add documentation

[Seth Jennings <sjenning@linux.vnet.ibm.com>]

This patch adds the documentation file for the zswap functionality

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
---
 Documentation/vm/zswap.txt | 72 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 Documentation/vm/zswap.txt

diff --git a/Documentation/vm/zswap.txt b/Documentation/vm/zswap.txt
new file mode 100644
index 0000000..88384b3
--- /dev/null
+++ b/Documentation/vm/zswap.txt
@@ -0,0 +1,72 @@
+Overview:
+
+Zswap is a lightweight compressed cache for swap pages. It takes pages that are
+in the process of being swapped out and attempts to compress them into a
+dynamically allocated RAM-based memory pool.  If this process is successful,
+the writeback to the swap device is deferred and, in many cases, avoided
+completely.A  This results in a significant I/O reduction and performance gains
+for systems that are swapping.
+
+Zswap provides compressed swap caching that basically trades CPU cycles for
+reduced swap I/O.A  This trade-off can result in a significant performance
+improvement as reads to/writes from to the compressed cache almost always
+faster that reading from a swap device which incurs the latency of an
+asynchronous block I/O read.
+
+Some potential benefits:
+* Desktop/laptop users with limited RAM capacities can mitigate the
+A A A  performance impact of swapping.
+* Overcommitted guests that share a common I/O resource can
+A A A  dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
+    throttling by the hypervisor.A This allows more work to get done with less
+    impact to the guest workload and guests sharing the I/O subsystem
+* Users with SSDs as swap devices can extend the life of the device by
+A A A  drastically reducing life-shortening writes.
+
+Zswap evicts pages from compressed cache on an LRU basis to the backing swap
+device when the compressed pool reaches it 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
+
+Design:
+
+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
+return 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.
+
+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
+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.
+
+During a page fault on a PTE that is a swap entry, frontswap calls the zswap
+load function to decompress the page into the page allocated by the page fault
+handler.
+
+Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
+in the swap_map goes to 0) the swap code calls the zswap invalidate function,
+via frontswap, to free the compressed entry.
+
+Zswap seeks to be simple in its policies.  Sysfs attributes allow for two user
+controlled policies:
+* max_compression_ratio - Maximum compression ratio, as as percentage,
+    for an acceptable compressed page. Any page that does not compress by at
+    least this ratio will be rejected.
+* 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
+a??compressora?? attribute.  The default compressor is lzo.  e.g.
+zswap.compressor=deflate
+
+A debugfs interface is provided for various statistic about pool size, number
+of pages stored, and various counters for the reasons pages are rejected.
-- 
1.8.2.2

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