mm: add SHRINK_EMPTY shrinker methods return value

We need to distinguish the situations when shrinker has very small
amount of objects (see vfs_pressure_ratio() called from
super_cache_count()), and when it has no objects at all.  Currently, in
the both of these cases, shrinker::count_objects() returns 0.

The patch introduces new SHRINK_EMPTY return value, which will be used
for "no objects at all" case.  It's is a refactoring mostly, as
SHRINK_EMPTY is replaced by 0 by all callers of do_shrink_slab() in this
patch, and all the magic will happen in further.

Link: http://lkml.kernel.org/r/153063069574.1818.11037751256699341813.stgit@localhost.localdomain
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Tested-by: Shakeel Butt <shakeelb@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Sahitya Tummala <stummala@codeaurora.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2aa3cb760189..8199e1b9a204 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -456,8 +456,8 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
         long scanned = 0, next_deferred;
 
         freeable = shrinker->count_objects(shrinker, shrinkctl);
-        if (freeable == 0)
-                return 0;
+        if (freeable == 0 || freeable == SHRINK_EMPTY)
+                return freeable;
 
         /*
          * copy the current shrinker scan count into a local variable
@@ -596,6 +596,8 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
                         continue;
 
                 ret = do_shrink_slab(&sc, shrinker, priority);
+                if (ret == SHRINK_EMPTY)
+                        ret = 0;
                 freed += ret;
 
                 if (rwsem_is_contended(&shrinker_rwsem)) {
@@ -641,6 +643,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 {
         struct shrinker *shrinker;
         unsigned long freed = 0;
+        int ret;
 
         if (!mem_cgroup_is_root(memcg))
                 return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
@@ -658,7 +661,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
                 if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
                         sc.nid = 0;
 
-                freed += do_shrink_slab(&sc, shrinker, priority);
+                ret = do_shrink_slab(&sc, shrinker, priority);
+                if (ret == SHRINK_EMPTY)
+                        ret = 0;
+                freed += ret;
                 /*
                  * Bail out if someone want to register a new shrinker to
                  * prevent the regsitration from being stalled for long periods