zsmalloc: move zspage object freeing into separate function

Move the code that frees a zspage object out of the zs_free()
function and into its own obj_free() function.

This is required by zsmalloc shrinking, which will also need to
free objects during zspage reclaiming.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
zsmalloc: add atomic index to find zspage to reclaim

Add an atomic index that allows multiple threads to concurrently and
sequentially iterate through the zspages in all classes and fullness
groups.  Add a function find_next_lru_class_fg() to find the next class
fullness group to check for a zspage.  Add a function find_lru_zspage()
which calls find_next_lru_class_fg() until a fullness group with an
available zspage is found.  This is required to implement zsmalloc pool
shrinking, which needs to be able to find a zspage to reclaim.

Since zsmalloc categorizes its zspages in arrays of fullness groups, which
are themselves inside arrays of classes, there is no (simple) way to
determine the LRU order of all a zsmalloc pool's zspages.  But to implement
shrinking, there must be some way to select the zspage to reclaim.  This
can't use a simple iteration through all classes, since any failure to
reclaim a zspage would result in any following reclaims to attempt to
reclaim the same zspage, which would likely result in repeated failures
to shrink the pool.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
zsmalloc: add zs_ops to zs_pool

Add struct zs_ops with a evict() callback function.  Add documentation
to zs_free() function clarifying that it cannot be called with a
zs_pool handle after that handle has been successfully evicted;
since evict calls into a function provided by the zs_pool creator,
the creator is therefore responsible for ensuring this requirement.

This is required to implement zsmalloc shrinking.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
zsmalloc: add obj_handle_is_free() function

Add function obj_handle_is_free() which scans through the entire
singly-linked list of free objects inside the provided zspage to
determine if the provided object handle is free or not.  This is
required by zspage reclaiming, which needs to evict each object
that is currently in use by the zs_pool owner, but has no other
way to determine if an object is in use.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
zsmalloc: add reclaim_zspage()

Add function reclaim_zspage() to evict each object in use in the provided
zspage, so that it can be freed.  This is required to be able to shrink
the zs_pool.  Check in zs_free() if the handle's zspage is in the reclaim
fullness group, and if so ignore it, since it will be freed during reclaim.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
zsmalloc: add zs_shrink()

Add function zs_shrink() to implement zs_pool shrinking.  This allows
the pool owner to reduce the size of the zs_pool by one zspage, which
contains one or more struct pages.  Once the zs_pool is shrunk, the
freed pages are available for system use.

This is used by zswap to limit its total system memory usage to a
user-defined amount, while attempting to keep the most recently stored
pages compressed in memory, and the oldest (or older) pages are evicted
from the zsmalloc zs_pool and written out to swap disk.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
zsmalloc: Connect zs_zpool_shrink() to zs_shrink()

Implement the zs_zpool_shrink() function, which previously just returned
EINVAL, by calling the zs_shrink() function to shrink the zs_pool by one
zspage.  The zs_shrink() function is called in a loop until the requested
number of pages have been reclaimed, or an error occurs.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>