zram: add size class equals check into recompression

It makes no sense for us to recompress the object if it will be in the
same size class. We anyway don't get any memory gain. But, at the same
time, we get a CPU time overhead when inserting this object into zspage
and decompressing it afterwards.

[senozhatsky: rebased and fixed conflicts]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Alexey Romanov <[e-mail]>
Signed-off-by: Sergey Senozhatsky <[e-mail]>
Acked-by: Minchan Kim <[e-mail]>
Cc: Nhat Pham <[e-mail]>
Cc: Nitin Gupta <[e-mail]>
Cc: Suleiman Souhlal <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/block/zram/zram_drv.c

···
 	unsigned long handle_new;
 	unsigned int comp_len_old;
 	unsigned int comp_len_new;
+	unsigned int class_index_old;
+	unsigned int class_index_new;
 	void *src, *dst;
 	int ret;
 
···
 	if (ret)
 		return ret;
 
+	class_index_old = zs_lookup_class_index(zram->mem_pool, comp_len_old);
 	/*
 	 * Iterate the secondary comp algorithms list (in order of priority)
 	 * and try to recompress the page.
···
 			return ret;
 		}
 
+		class_index_new = zs_lookup_class_index(zram->mem_pool,
+							comp_len_new);
+
 		/* Continue until we make progress */
 		if (comp_len_new >= huge_class_size ||
 		    comp_len_new >= comp_len_old ||
+		    class_index_new >= class_index_old ||
 		    (threshold && comp_len_new >= threshold)) {
 			zcomp_stream_put(zram->comps[prio]);
 			continue;
···
 	 * that would save memory, mark the object as incompressible so that
 	 * we will not try to compress it again.
 	 */
-	if (comp_len_new >= huge_class_size || comp_len_new >= comp_len_old) {
+	if (comp_len_new >= huge_class_size ||
+	    comp_len_new >= comp_len_old ||
+	    class_index_new >= class_index_old) {
 		zram_set_flag(zram, index, ZRAM_INCOMPRESSIBLE);
 		return 0;
 	}

include/linux/zsmalloc.h

···
 unsigned long zs_get_total_pages(struct zs_pool *pool);
 unsigned long zs_compact(struct zs_pool *pool);
 
+unsigned int zs_lookup_class_index(struct zs_pool *pool, unsigned int size);
+
 void zs_pool_stats(struct zs_pool *pool, struct zs_pool_stats *stats);
 #endif

mm/zsmalloc.c

···
 	return get_zspage_inuse(zspage) == class->objs_per_zspage;
 }
 
+/**
+ * zs_lookup_class_index() - Returns index of the zsmalloc &size_class
+ * that hold objects of the provided size.
+ * @pool: zsmalloc pool to use
+ * @size: object size
+ *
+ * Context: Any context.
+ *
+ * Return: the index of the zsmalloc &size_class that hold objects of the
+ * provided size.
+ */
+unsigned int zs_lookup_class_index(struct zs_pool *pool, unsigned int size)
+{
+	struct size_class *class;
+
+	class = pool->size_class[get_size_class_index(size)];
+
+	return class->index;
+}
+EXPORT_SYMBOL_GPL(zs_lookup_class_index);
+
 unsigned long zs_get_total_pages(struct zs_pool *pool)
 {
 	return atomic_long_read(&pool->pages_allocated);