[Devel] [PATCH RHEL8 COMMIT] mm: add SHRINK_EMPTY shrinker methods return value

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-80.1.2.vz8.3.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-80.1.2.vz8.3.4
------>
commit e623d6f386cf838121db016463a296757801a3b3
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Thu Apr 2 17:13:10 2020 +0300

    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 at virtuozzo.com>
    
    Acked-by: Vladimir Davydov <vdavydov.dev at gmail.com>
    Tested-by: Shakeel Butt <shakeelb at google.com>
    Cc: Al Viro <viro at zeniv.linux.org.uk>
    Cc: Andrey Ryabinin <aryabinin at virtuozzo.com>
    Cc: Chris Wilson <chris at chris-wilson.co.uk>
    Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
    Cc: Guenter Roeck <linux at roeck-us.net>
    Cc: "Huang, Ying" <ying.huang at intel.com>
    Cc: Johannes Weiner <hannes at cmpxchg.org>
    Cc: Josef Bacik <jbacik at fb.com>
    Cc: Li RongQing <lirongqing at baidu.com>
    Cc: Matthew Wilcox <willy at infradead.org>
    Cc: Matthias Kaehlcke <mka at chromium.org>
    Cc: Mel Gorman <mgorman at techsingularity.net>
    Cc: Michal Hocko <mhocko at kernel.org>
    Cc: Minchan Kim <minchan at kernel.org>
    Cc: Philippe Ombredanne <pombredanne at nexb.com>
    Cc: Roman Gushchin <guro at fb.com>
    Cc: Sahitya Tummala <stummala at codeaurora.org>
    Cc: Stephen Rothwell <sfr at canb.auug.org.au>
    Cc: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Cc: Waiman Long <longman at redhat.com>
    Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    (cherry picked from commit 9b996468cfdba09f688f52dba4287de596194613)
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 fs/super.c               |  3 +++
 include/linux/shrinker.h |  7 +++++--
 mm/vmscan.c              | 12 +++++++++---
 mm/workingset.c          |  3 +++
 4 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 3de936dc49e8..86eb13b92350 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -167,6 +167,9 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 	total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
 	total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);
 
+	if (!total_objects)
+		return SHRINK_EMPTY;
+
 	total_objects = vfs_pressure_ratio(total_objects);
 	return total_objects;
 }
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index cec7a37e8193..d720440f8fc6 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -36,12 +36,15 @@ struct shrink_control {
 };
 
 #define SHRINK_STOP (~0UL)
+#define SHRINK_EMPTY (~0UL - 1)
 /*
  * A callback you can register to apply pressure to ageable caches.
  *
  * @count_objects should return the number of freeable items in the cache. If
- * there are no objects to free or the number of freeable items cannot be
- * determined, it should return 0. No deadlock checks should be done during the
+ * there are no objects to free, it should return SHRINK_EMPTY, while 0 is
+ * returned in cases of the number of freeable items cannot be determined
+ * or shrinker should skip this cache for this time (e.g., their number
+ * is below shrinkable limit). No deadlock checks should be done during the
  * count callback - the shrinker relies on aggregating scan counts that couldn't
  * be executed due to potential deadlocks to be run at a later call when the
  * deadlock condition is no longer pending.
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0395e910f424..45d116cc62cd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -424,8 +424,8 @@ 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
@@ -564,6 +564,8 @@ 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)) {
@@ -609,6 +611,7 @@ 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);
@@ -630,7 +633,10 @@ 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
diff --git a/mm/workingset.c b/mm/workingset.c
index c3a7a9f58428..fb6321ddd1b7 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -466,6 +466,9 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
 	}
 	max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3);
 
+	if (!nodes)
+		return SHRINK_EMPTY;
+
 	if (nodes <= max_nodes)
 		return 0;
 	return nodes - max_nodes;