This patchset tweaks compaction and makes it possible to trigger pool
compaction automatically when system is getting low on memory.

zsmalloc in some cases can suffer from a notable fragmentation and
compaction can release some considerable amount of memory.  The problem
here is that currently we fully rely on user space to perform compaction
when needed.  However, performing zsmalloc compaction is not always an
obvious thing to do.  For example, suppose we have a `idle' fragmented
(compaction was never performed) zram device and system is getting low
on memory due to some 3rd party user processes (gcc LTO, or firefox,
etc.).  It's quite unlikely that user space will issue zpool compaction
in this case.  Besides, user space cannot tell for sure how badly pool
is fragmented; however, this info is known to zsmalloc and, hence, to a
shrinker.

This patch (of 7):

__zs_compact() does not use `nr_to_migrate', drop it.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>