brd: expose number of allocated pages in debugfs

While the maximum size of each ramdisk is defined either as a module
parameter, or compile time default, it's impossible to know how many pages
have currently been allocated by each ram%d device, since they're
allocated when used and never freed.

This patch creates a new directory at this location:

/sys/kernel/debug/ramdisk_pages/

which will contain a file named "ram%d" for each instantiated ramdisk on
the system. The file is read-only, and read() will output the number of
pages currently held by that ramdisk.

We lose track how much memory a ramdisk is using as pages once used are
simply recycled but never freed.

In instances where we exhaust the size of the ramdisk with a file that
exceeds it, encounter ENOSPC and delete the file for mitigation; df would
show decrease in used and increase in available blocks but the since we
have touched all pages, the memory footprint of the ramdisk does not
reflect the blocks used/available count

...
[root@localhost ~]# mkfs.ext2 /dev/ram15
mke2fs 1.45.6 (20-Mar-2020)
Creating filesystem with 4096 1k blocks and 1024 inodes
[root@localhost ~]# mount /dev/ram15 /mnt/ram15/

[root@localhost ~]# cat
/sys/kernel/debug/ramdisk_pages/ram15
58
[root@kerneltest008.06.prn3 ~]# df /dev/ram15
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/ram15          3963    31      3728   1% /mnt/ram15
[root@kerneltest008.06.prn3 ~]# dd if=/dev/urandom of=/mnt/ram15/test2
bs=1M count=5
dd: error writing '/mnt/ram15/test2': No space left on device
4+0 records in
3+0 records out
4005888 bytes (4.0 MB, 3.8 MiB) copied, 0.0446614 s, 89.7 MB/s
[root@kerneltest008.06.prn3 ~]# df /mnt/ram15/
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/ram15          3963  3960         0 100% /mnt/ram15
[root@kerneltest008.06.prn3 ~]# cat
/sys/kernel/debug/ramdisk_pages/ram15
1024
[root@kerneltest008.06.prn3 ~]# rm /mnt/ram15/test2
rm: remove regular file '/mnt/ram15/test2'? y
[root@kerneltest008.06.prn3 /var]# df /dev/ram15
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/ram15          3963    31      3728   1% /mnt/ram15

# Acutal memory footprint
[root@kerneltest008.06.prn3 /var]# cat
/sys/kernel/debug/ramdisk_pages/ram15
1024
...

This debugfs counter will always reveal the accurate number of
permanently allocated pages to the ramdisk.

Signed-off-by: Calvin Owens <calvinowens@fb.com>
[cleaned up the !CONFIG_DEBUG_FS case and API changes for HEAD]
Signed-off-by: Kyle McMartin <jkkm@fb.com>
[rebased]
Signed-off-by: Saravanan D <saravanand@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/brd.c

@@ -22,6 +22,7 @@
 #include <linux/fs.h>
 #include <linux/slab.h>
 #include <linux/backing-dev.h>
+#include <linux/debugfs.h>
 
 #include <linux/uaccess.h>
 
@@ -48,6 +49,7 @@ struct brd_device {
 	 */
 	spinlock_t		brd_lock;
 	struct radix_tree_root	brd_pages;
+	u64			brd_nr_pages;
 };
 
 /*
@@ -116,6 +118,8 @@ static struct page *brd_insert_page(struct brd_device *brd, sector_t sector)
 		page = radix_tree_lookup(&brd->brd_pages, idx);
 		BUG_ON(!page);
 		BUG_ON(page->index != idx);
+	} else {
+		brd->brd_nr_pages++;
 	}
 	spin_unlock(&brd->brd_lock);
 
@@ -365,11 +369,13 @@ __setup("ramdisk_size=", ramdisk_size);
  */
 static LIST_HEAD(brd_devices);
 static DEFINE_MUTEX(brd_devices_mutex);
+static struct dentry *brd_debugfs_dir;
 
 static struct brd_device *brd_alloc(int i)
 {
 	struct brd_device *brd;
 	struct gendisk *disk;
+	char buf[DISK_NAME_LEN];
 
 	brd = kzalloc(sizeof(*brd), GFP_KERNEL);
 	if (!brd)
@@ -382,6 +388,11 @@ static struct brd_device *brd_alloc(int i)
 	if (!brd->brd_queue)
 		goto out_free_dev;
 
+	snprintf(buf, DISK_NAME_LEN, "ram%d", i);
+	if (!IS_ERR_OR_NULL(brd_debugfs_dir))
+		debugfs_create_u64(buf, 0444, brd_debugfs_dir,
+				&brd->brd_nr_pages);
+
 	/* This is so fdisk will align partitions on 4k, because of
 	 * direct_access API needing 4k alignment, returning a PFN
 	 * (This is only a problem on very small devices <= 4M,
@@ -397,7 +408,7 @@ static struct brd_device *brd_alloc(int i)
 	disk->fops		= &brd_fops;
 	disk->private_data	= brd;
 	disk->flags		= GENHD_FL_EXT_DEVT;
-	sprintf(disk->disk_name, "ram%d", i);
+	strlcpy(disk->disk_name, buf, DISK_NAME_LEN);
 	set_capacity(disk, rd_size * 2);
 
 	/* Tell the block layer that this is not a rotational device */
@@ -495,6 +506,8 @@ static int __init brd_init(void)
 
 	brd_check_and_reset_par();
 
+	brd_debugfs_dir = debugfs_create_dir("ramdisk_pages", NULL);
+
 	mutex_lock(&brd_devices_mutex);
 	for (i = 0; i < rd_nr; i++) {
 		brd = brd_alloc(i);
@@ -519,6 +532,8 @@ static int __init brd_init(void)
 	return 0;
 
 out_free:
+	debugfs_remove_recursive(brd_debugfs_dir);
+
 	list_for_each_entry_safe(brd, next, &brd_devices, brd_list) {
 		list_del(&brd->brd_list);
 		brd_free(brd);
@@ -534,6 +549,8 @@ static void __exit brd_exit(void)
 {
 	struct brd_device *brd, *next;
 
+	debugfs_remove_recursive(brd_debugfs_dir);
+
 	list_for_each_entry_safe(brd, next, &brd_devices, brd_list)
 		brd_del_one(brd);