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.  zswap basically trades CPU cycles
  for potentially reduced swap I/O.  This trade-off can also result in a
  significant performance improvement if reads from the compressed cache are
  faster than reads from a swap device.
  
  NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory
  reclaim.  This interaction has not been fully explored on the large set of
  potential configurations and workloads that exist.  For this reason, zswap
  is a work in progress and should be considered experimental.
  
  Some potential benefits:
  * Desktop/laptop users with limited RAM capacities can mitigate the
      performance impact of swapping.
  * Overcommitted guests that share a common I/O resource can
      dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
      throttling by the hypervisor. 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
      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 its size limit.  This requirement had
  been identified in prior community discussions.
  
  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:
  
  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 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.  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 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.
  
  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 one user
  controlled policy:
  * max_pool_percent - The maximum percentage of memory that the compressed
      pool can occupy.
  
  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.