[PATCH] brd: expose number of allocated pages in debugfs

[Kyle McMartin]

From: Calvin Owens <calvinow...@fb.com>

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.

Signed-off-by: Calvin Owens <calvinow...@fb.com>
[cleaned up the !CONFIG_DEBUG_FS case and API changes for HEAD]
Signed-off-by: Kyle McMartin <j...@fb.com>
---
 drivers/block/brd.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index 2723a70eb855..39b9d6aee5b8 100644
--- a/drivers/block/brd.c
+++ b/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);
 
@@ -369,11 +373,13 @@ __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)
@@ -386,6 +392,11 @@ 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,
@@ -401,7 +412,7 @@ 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);
        brd->brd_queue->backing_dev_info->capabilities |= 
BDI_CAP_SYNCHRONOUS_IO;
 
@@ -516,6 +527,8 @@ static int __init brd_init(void)
 
        brd_check_and_reset_par();
 
+       brd_debugfs_dir = debugfs_create_dir("ramdisk_pages", NULL);
+
        for (i = 0; i < rd_nr; i++) {
                brd = brd_alloc(i);
                if (!brd)
@@ -541,6 +554,8 @@ 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);
@@ -555,6 +570,8 @@ 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);
 
-- 
2.26.2