[PATCH 09/10] zsmalloc: add zs_shrink()

[Dan Streetman]

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@xxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
---
 include/linux/zsmalloc.h |  1 +
 mm/zsmalloc.c            | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index 2c341d4..07fe84d 100644
--- a/include/linux/zsmalloc.h
+++ b/include/linux/zsmalloc.h
@@ -45,6 +45,7 @@ void zs_destroy_pool(struct zs_pool *pool);
 
 unsigned long zs_malloc(struct zs_pool *pool, size_t size);
 void zs_free(struct zs_pool *pool, unsigned long obj);
+int zs_shrink(struct zs_pool *pool);
 
 void *zs_map_object(struct zs_pool *pool, unsigned long handle,
 			enum zs_mapmode mm);
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 60fd23e..f769c21 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1296,6 +1296,41 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
 EXPORT_SYMBOL_GPL(zs_free);
 
 /**
+ * zs_shrink - Shrink the pool
+ * @pool: pool to shrink
+ *
+ * The pool will be shrunk by one zspage, which is some
+ * number of pages in size.  On success, the number of freed
+ * pages is returned.  On failure, the error is returned.
+ */
+int zs_shrink(struct zs_pool *pool)
+{
+	struct size_class *class;
+	enum fullness_group fullness;
+	struct page *page;
+	int class_idx, ret;
+
+	if (!pool->ops || !pool->ops->evict)
+		return -EINVAL;
+
+	/* if a page is found, the class is locked */
+	page = find_lru_zspage(pool);
+	if (!page)
+		return -ENOENT;
+
+	get_zspage_mapping(page, &class_idx, &fullness);
+	class = &pool->size_class[class_idx];
+
+	/* reclaim_zspage unlocks the class lock */
+	ret = reclaim_zspage(pool, page);
+	if (ret)
+		return ret;
+
+	return class->pages_per_zspage;
+}
+EXPORT_SYMBOL_GPL(zs_shrink);
+
+/**
  * zs_map_object - get address of allocated object from handle.
  * @pool: pool from which the object was allocated
  * @handle: handle returned from zs_malloc
-- 
1.8.3.1

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