[PATCH v2] zram: add restriction on dynamic zram device creation

[Yi Wang <wang.yi59@xxxxxxxxxx>]

From: zhanglin <zhang.lin16@xxxxxxxxxx>

Add max_num_devices to limit dynamic zram device creation to prevent
 potential OOM

Signed-off-by: zhanglin <zhang.lin16@xxxxxxxxxx>
Signed-off-by: Yi Wang <wang.yi59@xxxxxxxxxx>
---
v1->v2:
change hard-coded initial max_num_devices into configurable way.

 drivers/block/zram/Kconfig    |  7 +++++++
 drivers/block/zram/zram_drv.c | 28 +++++++++++++++++++++-------
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
index fe7a4b7d30cf..54a369932417 100644
--- a/drivers/block/zram/Kconfig
+++ b/drivers/block/zram/Kconfig
@@ -37,3 +37,10 @@ config ZRAM_MEMORY_TRACKING
 	  /sys/kernel/debug/zram/zramX/block_state.
 
 	  See Documentation/admin-guide/blockdev/zram.rst for more information.
+
+config ZRAM_DEV_MAX_COUNT
+	int "Number of zram devices to be created"
+	depends on ZRAM
+	default 256
+	help
+	  This option specifies the maximum number of zram devices.
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 36d49159140f..d1022f3c04c4 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -43,8 +43,9 @@ static DEFINE_MUTEX(zram_index_mutex);
 static int zram_major;
 static const char *default_compressor = "lzo-rle";
 
-/* Module params (documentation at end) */
 static unsigned int num_devices = 1;
+/* Module params (documentation at end) */
+static unsigned int max_num_devices = CONFIG_ZRAM_DEV_MAX_COUNT;
 /*
  * Pages that compress to sizes equals or greater than this are stored
  * uncompressed in memory.
@@ -2013,10 +2014,16 @@ static ssize_t hot_add_show(struct class *class,
 			struct class_attribute *attr,
 			char *buf)
 {
-	int ret;
+	int ret = -ENOSPC;
 
 	mutex_lock(&zram_index_mutex);
+	if (num_devices >= max_num_devices) {
+		mutex_unlock(&zram_index_mutex);
+		return ret;
+	}
 	ret = zram_add();
+	if (ret >= 0)
+		num_devices += 1;
 	mutex_unlock(&zram_index_mutex);
 
 	if (ret < 0)
@@ -2046,8 +2053,10 @@ static ssize_t hot_remove_store(struct class *class,
 	zram = idr_find(&zram_index_idr, dev_id);
 	if (zram) {
 		ret = zram_remove(zram);
-		if (!ret)
+		if (!ret) {
 			idr_remove(&zram_index_idr, dev_id);
+			num_devices -= 1;
+		}
 	} else {
 		ret = -ENODEV;
 	}
@@ -2089,6 +2098,7 @@ static void destroy_devices(void)
 static int __init zram_init(void)
 {
 	int ret;
+	unsigned int i;
 
 	ret = cpuhp_setup_state_multi(CPUHP_ZCOMP_PREPARE, "block/zram:prepare",
 				      zcomp_cpu_up_prepare, zcomp_cpu_dead);
@@ -2111,13 +2121,17 @@ static int __init zram_init(void)
 		return -EBUSY;
 	}
 
-	while (num_devices != 0) {
+	if (num_devices > max_num_devices) {
+		pr_err("Number of pre-created zram devices over limit\n");
+		goto out_error;
+	}
+
+	for (i = 0; i < num_devices; i++) {
 		mutex_lock(&zram_index_mutex);
 		ret = zram_add();
 		mutex_unlock(&zram_index_mutex);
 		if (ret < 0)
 			goto out_error;
-		num_devices--;
 	}
 
 	return 0;
@@ -2135,8 +2149,8 @@ static void __exit zram_exit(void)
 module_init(zram_init);
 module_exit(zram_exit);
 
-module_param(num_devices, uint, 0);
-MODULE_PARM_DESC(num_devices, "Number of pre-created zram devices");
+module_param(max_num_devices, uint, 0);
+MODULE_PARM_DESC(max_num_devices, "Max number of created zram devices");
 
 MODULE_LICENSE("Dual BSD/GPL");
 MODULE_AUTHOR("Nitin Gupta <ngupta@xxxxxxxxxx>");
-- 
2.17.1