[PATCHv6 8/8] zswap: add documentation

[Seth Jennings]

This patch adds the documentation file for the zswap functionality

Signed-off-by: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx>
---
 Documentation/vm/zswap.txt | 82 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 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..f29b82f
--- /dev/null
+++ b/Documentation/vm/zswap.txt
@@ -0,0 +1,82 @@
+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. 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. 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
+ÂÂÂ performance impact of swapping.
+* Overcommitted guests that share a common I/O resource can
+ÂÂÂ dramatically reduce their swap I/O pressure, avoiding heavy
+ handed I/O throttling by the hypervisor. This allows more work
+ÂÂÂ to get done with less impact to the guest workload and guests
+ÂÂÂ sharing the I/O subsystem
+* Users with SSDs as swap devices can extend the life of the device by
+ÂÂÂ drastically reducing life-shortening writes.
+
+Zswap evicts pages from compressed cache on an LRU basis to the backing
+swap device when the compress pool reaches it size limit or the pool is
+unable to obtain additional pages from the buddy allocator. 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 or unable to secure additional pages from
+the buddy allocator.
+
+Zswap makes use of zsmalloc for the managing the compressed memory
+pool.  This is because zsmalloc is specifically designed to minimize
+fragmentation on large (> PAGE_SIZE/2) allocation sizes.  Each
+allocation in zsmalloc 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 zsmalloc 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 âcompressorâ 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.
-- 
1.8.1.1

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