zsmalloc
  --------
  
  This allocator is designed for use with zram. Thus, the allocator is
  supposed to work well under low memory conditions. In particular, it
  never attempts higher order page allocation which is very likely to
  fail under memory pressure. On the other hand, if we just use single
  (0-order) pages, it would suffer from very high fragmentation --
  any object of size PAGE_SIZE/2 or larger would occupy an entire page.
  This was one of the major issues with its predecessor (xvmalloc).
  
  To overcome these issues, zsmalloc allocates a bunch of 0-order pages
  and links them together using various 'struct page' fields. These linked
  pages act as a single higher-order page i.e. an object can span 0-order
  page boundaries. The code refers to these linked pages as a single entity
  called zspage.
  
  For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
  since this satisfies the requirements of all its current users (in the
  worst case, page is incompressible and is thus stored "as-is" i.e. in
  uncompressed form). For allocation requests larger than this size, failure
  is returned (see zs_malloc).
  
  Additionally, zs_malloc() does not return a dereferenceable pointer.
  Instead, it returns an opaque handle (unsigned long) which encodes actual
  location of the allocated object. The reason for this indirection is that
  zsmalloc does not keep zspages permanently mapped since that would cause
  issues on 32-bit systems where the VA region for kernel space mappings
  is very small. So, before using the allocating memory, the object has to
  be mapped using zs_map_object() to get a usable pointer and subsequently
  unmapped using zs_unmap_object().
  
  stat
  ----
  
  With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via
  /sys/kernel/debug/zsmalloc/<user name>. Here is a sample of stat output:
  
  # cat /sys/kernel/debug/zsmalloc/zram0/classes
  
   class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage
      ..
      ..
       9   176           0            1           186        129          8                4
      10   192           1            0          2880       2872        135                3
      11   208           0            1           819        795         42                2
      12   224           0            1           219        159         12                4
      ..
      ..
  
  
  class: index
  size: object size zspage stores
  almost_empty: the number of ZS_ALMOST_EMPTY zspages(see below)
  almost_full: the number of ZS_ALMOST_FULL zspages(see below)
  obj_allocated: the number of objects allocated
  obj_used: the number of objects allocated to the user
  pages_used: the number of pages allocated for the class
  pages_per_zspage: the number of 0-order pages to make a zspage
  
  We assign a zspage to ZS_ALMOST_EMPTY fullness group when:
        n <= N / f, where
  n = number of allocated objects
  N = total number of objects zspage can store
  f = fullness_threshold_frac(ie, 4 at the moment)
  
  Similarly, we assign zspage to:
        ZS_ALMOST_FULL  when n > N / f
        ZS_EMPTY        when n == 0
        ZS_FULL         when n == N