From e04909aac2425074d684bac70563b87d462982fe Mon Sep 17 00:00:00 2001 From: "Barry Song (Xiaomi)" Date: Sun, 12 Apr 2026 14:04:50 +0800 Subject: [PATCH] zram: support asynchronous GC for lazy slot freeing MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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/20240805153639.1057-1-justinjiang@vivo.com/ [2] https://lore.kernel.org/all/20250909065349.574894-1-liulei.rjpt@vivo.com/ Tested-by: Xueyuan Chen Signed-off-by: Barry Song (Xiaomi) --- drivers/block/zram/zram_drv.c | 56 ++++++++++++++++++++++++++++++++++- drivers/block/zram/zram_drv.h | 3 ++ 2 files changed, 58 insertions(+), 1 deletion(-) diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c index aebc710f0d6a..718a3a64c4bd 100644 --- a/drivers/block/zram/zram_drv.c +++ b/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; @@ -1966,6 +1983,9 @@ 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); @@ -1973,6 +1993,14 @@ static void zram_meta_free(struct zram *zram, u64 disksize) 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); } diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h index 08d1774c15db..1f3ffd79fcb1 100644 --- a/drivers/block/zram/zram_drv.h +++ b/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