Commit f4be591f authored Apr 16, 2021 by Calvin Owens Committed by Jens Axboe Apr 21, 2021

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>

parent b777f4c4

drivers/block/brd.c

+18 −1

Original line number	Diff line number	Diff line
		@@ -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);