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Subject[PATCH v9 16/17] From: Kirill Tkhai <>
From
Date
mm: Add SHRINK_EMPTY shrinker methods return value

We need to differ 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.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Tested-by: Shakeel Butt <shakeelb@google.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 f5f96e52e0cd..7429588d6b49 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -144,6 +144,9 @@ 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 7ca9c18cf130..b154fd2b084c 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -34,12 +34,15 @@ 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 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
diff --git a/mm/workingset.c b/mm/workingset.c
index cd0b2ae615e4..bc72ad029b3e 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -399,6 +399,9 @@ 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;
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