zram: support asynchronous GC for lazy slot freeing

Swap freeing can be expensive when unmapping a VMA containing
many swap entries. This has been reported to significantly
delay memory reclamation during Android’s low-memory killing,
especially when multiple processes are terminated to free
memory, with slot_free() accounting for more than 80% of
the total cost of freeing swap entries.

Two earlier attempts by Lei and Zhiguo added a new thread in the mm core
to asynchronously collect and free swap entries [1][2], but the
design itself is fairly complex.

When anon folios and swap entries are mixed within a
process, reclaiming anon folios from killed processes
helps return memory to the system as quickly as possible,
so that newly launched applications can satisfy their
memory demands. It is not ideal for swap freeing to block
anon folio freeing. On the other hand, swap freeing can
still return memory to the system, although at a slower
rate due to memory compression.

Therefore, in zram, we introduce a GC worker to allow anon
folio freeing and slot_free to run in parallel, since
slot_free is performed asynchronously, maximizing the rate at
which memory is returned to the system.

Xueyuan’s test on RK3588 shows that unmapping a 256MB swap-filled
VMA becomes 3.4× faster when pinning tasks to CPU2, reducing the
execution time from 63,102,982 ns to 18,570,726 ns.

A positive side effect is that async GC also slightly improves
do_swap_page() performance, as it no longer has to wait for
slot_free() to complete.

Xueyuan’s test shows that swapping in 256MB of data (each page
filled with repeating patterns such as “1024 one”, “1024 two”,
“1024 three”, and “1024 four”) reduces execution time from
1,358,133,886 ns to 1,104,315,986 ns, achieving a 1.22× speedup.

[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/all/[email protected]/

Tested-by: Xueyuan Chen <[email protected]>
Signed-off-by: Barry Song (Xiaomi) <[email protected]>

Author: Barry Song (Xiaomi)

drivers/block/zram/zram_drv.c

@@ -1958,6 +1958,23 @@ static ssize_t debug_stat_show(struct device *dev,
 	return ret;
 }
 
+static void gc_slots_free(struct zram *zram)
+{
+	size_t num_pages = zram->disksize >> PAGE_SHIFT;
+	unsigned long index;
+
+	index = find_next_bit(zram->gc_map, num_pages, 0);
+	while (index < num_pages) {
+		if (slot_trylock(zram, index)) {
+			if (test_bit(index, zram->gc_map))
+				slot_free(zram, index);
+			slot_unlock(zram, index);
+			cond_resched();
+		}
+		index = find_next_bit(zram->gc_map, num_pages, index + 1);
+	}
+}
+
 static void zram_meta_free(struct zram *zram, u64 disksize)
 {
 	size_t num_pages = disksize >> PAGE_SHIFT;
@@ -1966,13 +1983,24 @@ static void zram_meta_free(struct zram *zram, u64 disksize)
 	if (!zram->table)
 		return;
 
+	flush_work(&zram->gc_work);
+	gc_slots_free(zram);
+
 	/* Free all pages that are still in this zram device */
 	for (index = 0; index < num_pages; index++)
 		slot_free(zram, index);
 
 	zs_destroy_pool(zram->mem_pool);
 	vfree(zram->table);
 	zram->table = NULL;
+	bitmap_free(zram->gc_map);
+}
+
+static void zram_gc_work(struct work_struct *work)
+{
+	struct zram *zram = container_of(work, struct zram, gc_work);
+
+	gc_slots_free(zram);
 }
 
 static bool zram_meta_alloc(struct zram *zram, u64 disksize)
@@ -1991,12 +2019,22 @@ static bool zram_meta_alloc(struct zram *zram, u64 disksize)
 		return false;
 	}
 
+	zram->gc_map = bitmap_zalloc(num_pages, GFP_KERNEL);
+	if (!zram->gc_map) {
+		zs_destroy_pool(zram->mem_pool);
+		vfree(zram->table);
+		zram->table = NULL;
+		return false;
+	}
+
 	if (!huge_class_size)
 		huge_class_size = zs_huge_class_size(zram->mem_pool);
 
 	for (index = 0; index < num_pages; index++)
 		slot_lock_init(zram, index);
 
+	INIT_WORK(&zram->gc_work, zram_gc_work);
+
 	return true;
 }
 
@@ -2008,6 +2046,8 @@ static void slot_free(struct zram *zram, u32 index)
 	zram->table[index].attr.ac_time = 0;
 #endif
 
+	if (test_and_clear_bit(index, zram->gc_map))
+		atomic64_dec(&zram->stats.gc_slots);
 	clear_slot_flag(zram, index, ZRAM_IDLE);
 	clear_slot_flag(zram, index, ZRAM_INCOMPRESSIBLE);
 	clear_slot_flag(zram, index, ZRAM_PP_SLOT);
@@ -2784,6 +2824,19 @@ static void zram_submit_bio(struct bio *bio)
 	}
 }
 
+static bool try_slot_lazy_free(struct zram *zram, unsigned long index)
+{
+	/* too many lazy-free slots, perform direct free */
+	if (atomic64_read(&zram->stats.gc_slots) > 30000)
+		return false;
+	if (test_and_set_bit(index, zram->gc_map))
+		return false;
+	/* accumulated lazy-free slots, wake up GC worker */
+	if (atomic64_inc_return(&zram->stats.gc_slots) > 200)
+		queue_work(system_dfl_wq, &zram->gc_work);
+	return true;
+}
+
 static void zram_slot_free_notify(struct block_device *bdev,
 				unsigned long index)
 {
@@ -2797,7 +2850,8 @@ static void zram_slot_free_notify(struct block_device *bdev,
 		return;
 	}
 
-	slot_free(zram, index);
+	if (!try_slot_lazy_free(zram, index))
+		slot_free(zram, index);
 	slot_unlock(zram, index);
 }
 

drivers/block/zram/zram_drv.h

@@ -88,6 +88,7 @@ struct zram_stats {
 	atomic64_t pages_stored;	/* no. of pages currently stored */
 	atomic_long_t max_used_pages;	/* no. of maximum pages stored */
 	atomic64_t miss_free;		/* no. of missed free */
+	atomic64_t gc_slots;		/* no. of queued for lazy free by gc */
 #ifdef	CONFIG_ZRAM_WRITEBACK
 	atomic64_t bd_count;		/* no. of pages in backing device */
 	atomic64_t bd_reads;		/* no. of reads from backing device */
@@ -142,5 +143,7 @@ struct zram {
 #ifdef CONFIG_ZRAM_MEMORY_TRACKING
 	struct dentry *debugfs_dir;
 #endif
+	unsigned long *gc_map;
+	struct work_struct gc_work;
 };
 #endif