btrfs: fix wrong free space information of btrfs

When we store data by raid profile in btrfs with two or more different size
disks, df command shows there is some free space in the filesystem, but the
user can not write any data in fact, df command shows the wrong free space
information of btrfs.

 # mkfs.btrfs -d raid1 /dev/sda9 /dev/sda10
 # btrfs-show
 Label: none  uuid: a95cd49e-6e33-45b8-8741-a36153ce4b64
 	Total devices 2 FS bytes used 28.00KB
 	devid    1 size 5.01GB used 2.03GB path /dev/sda9
 	devid    2 size 10.00GB used 2.01GB path /dev/sda10
 # btrfs device scan /dev/sda9 /dev/sda10
 # mount /dev/sda9 /mnt
 # dd if=/dev/zero of=tmpfile0 bs=4K count=9999999999
   (fill the filesystem)
 # sync
 # df -TH
 Filesystem	Type	Size	Used	Avail	Use%	Mounted on
 /dev/sda9	btrfs	17G	8.6G	5.4G	62%	/mnt
 # btrfs-show
 Label: none  uuid: a95cd49e-6e33-45b8-8741-a36153ce4b64
 	Total devices 2 FS bytes used 3.99GB
 	devid    1 size 5.01GB used 5.01GB path /dev/sda9
 	devid    2 size 10.00GB used 4.99GB path /dev/sda10

It is because btrfs cannot allocate chunks when one of the pairing disks has
no space, the free space on the other disks can not be used for ever, and should
be subtracted from the total space, but btrfs doesn't subtract this space from
the total. It is strange to the user.

This patch fixes it by calcing the free space that can be used to allocate
chunks.

Implementation:
1. get all the devices free space, and align them by stripe length.
2. sort the devices by the free space.
3. check the free space of the devices,
   3.1. if it is not zero, and then check the number of the devices that has
        more free space than this device,
        if the number of the devices is beyond the min stripe number, the free
        space can be used, and add into total free space.
        if the number of the devices is below the min stripe number, we can not
        use the free space, the check ends.
   3.2. if the free space is zero, check the next devices, goto 3.1

This implementation is just likely fake chunk allocation.

After appling this patch, df can show correct space information:
 # df -TH
 Filesystem	Type	Size	Used	Avail	Use%	Mounted on
 /dev/sda9	btrfs	17G	8.6G	0	100%	/mnt

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 140 insertions, 6 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index caa5bcc62f16..2963376e77f4 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -777,6 +777,127 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
         return 0;
 }
 
+/*
+ * The helper to calc the free space on the devices that can be used to store
+ * file data.
+ */
+static int btrfs_calc_avail_data_space(struct btrfs_root *root, u64 *free_bytes)
+{
+        struct btrfs_fs_info *fs_info = root->fs_info;
+        struct btrfs_device_info *devices_info;
+        struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
+        struct btrfs_device *device;
+        u64 skip_space;
+        u64 type;
+        u64 avail_space;
+        u64 used_space;
+        u64 min_stripe_size;
+        int min_stripes = 1;
+        int i = 0, nr_devices;
+        int ret;
+
+        nr_devices = fs_info->fs_devices->rw_devices;
+        BUG_ON(!nr_devices);
+
+        devices_info = kmalloc(sizeof(*devices_info) * nr_devices,
+                               GFP_NOFS);
+        if (!devices_info)
+                return -ENOMEM;
+
+        /* calc min stripe number for data space alloction */
+        type = btrfs_get_alloc_profile(root, 1);
+        if (type & BTRFS_BLOCK_GROUP_RAID0)
+                min_stripes = 2;
+        else if (type & BTRFS_BLOCK_GROUP_RAID1)
+                min_stripes = 2;
+        else if (type & BTRFS_BLOCK_GROUP_RAID10)
+                min_stripes = 4;
+
+        if (type & BTRFS_BLOCK_GROUP_DUP)
+                min_stripe_size = 2 * BTRFS_STRIPE_LEN;
+        else
+                min_stripe_size = BTRFS_STRIPE_LEN;
+
+        list_for_each_entry(device, &fs_devices->alloc_list, dev_alloc_list) {
+                if (!device->in_fs_metadata)
+                        continue;
+
+                avail_space = device->total_bytes - device->bytes_used;
+
+                /* align with stripe_len */
+                do_div(avail_space, BTRFS_STRIPE_LEN);
+                avail_space *= BTRFS_STRIPE_LEN;
+
+                /*
+                 * In order to avoid overwritting the superblock on the drive,
+                 * btrfs starts at an offset of at least 1MB when doing chunk
+                 * allocation.
+                 */
+                skip_space = 1024 * 1024;
+
+                /* user can set the offset in fs_info->alloc_start. */
+                if (fs_info->alloc_start + BTRFS_STRIPE_LEN <=
+                    device->total_bytes)
+                        skip_space = max(fs_info->alloc_start, skip_space);
+
+                /*
+                 * btrfs can not use the free space in [0, skip_space - 1],
+                 * we must subtract it from the total. In order to implement
+                 * it, we account the used space in this range first.
+                 */
+                ret = btrfs_account_dev_extents_size(device, 0, skip_space - 1,
+                                                     &used_space);
+                if (ret) {
+                        kfree(devices_info);
+                        return ret;
+                }
+
+                /* calc the free space in [0, skip_space - 1] */
+                skip_space -= used_space;
+
+                /*
+                 * we can use the free space in [0, skip_space - 1], subtract
+                 * it from the total.
+                 */
+                if (avail_space && avail_space >= skip_space)
+                        avail_space -= skip_space;
+                else
+                        avail_space = 0;
+
+                if (avail_space < min_stripe_size)
+                        continue;
+
+                devices_info[i].dev = device;
+                devices_info[i].max_avail = avail_space;
+
+                i++;
+        }
+
+        nr_devices = i;
+
+        btrfs_descending_sort_devices(devices_info, nr_devices);
+
+        i = nr_devices - 1;
+        avail_space = 0;
+        while (nr_devices >= min_stripes) {
+                if (devices_info[i].max_avail >= min_stripe_size) {
+                        int j;
+                        u64 alloc_size;
+
+                        avail_space += devices_info[i].max_avail * min_stripes;
+                        alloc_size = devices_info[i].max_avail;
+                        for (j = i + 1 - min_stripes; j <= i; j++)
+                                devices_info[j].max_avail -= alloc_size;
+                }
+                i--;
+                nr_devices--;
+        }
+
+        kfree(devices_info);
+        *free_bytes = avail_space;
+        return 0;
+}
+
 static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
         struct btrfs_root *root = btrfs_sb(dentry->d_sb);
@@ -784,16 +905,21 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
         struct list_head *head = &root->fs_info->space_info;
         struct btrfs_space_info *found;
         u64 total_used = 0;
-        u64 total_used_data = 0;
+        u64 total_free_data = 0;
         int bits = dentry->d_sb->s_blocksize_bits;
         __be32 *fsid = (__be32 *)root->fs_info->fsid;
+        int ret;
 
+        /* holding chunk_muext to avoid allocating new chunks */
+        mutex_lock(&root->fs_info->chunk_mutex);
         rcu_read_lock();
         list_for_each_entry_rcu(found, head, list) {
-                if (found->flags & BTRFS_BLOCK_GROUP_DATA)
-                        total_used_data += found->disk_used;
-                else
-                        total_used_data += found->disk_total;
+                if (found->flags & BTRFS_BLOCK_GROUP_DATA) {
+                        total_free_data += found->disk_total - found->disk_used;
+                        total_free_data -=
+                                btrfs_account_ro_block_groups_free_space(found);
+                }
+
                 total_used += found->disk_used;
         }
         rcu_read_unlock();
@@ -801,9 +927,17 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
         buf->f_namelen = BTRFS_NAME_LEN;
         buf->f_blocks = btrfs_super_total_bytes(disk_super) >> bits;
         buf->f_bfree = buf->f_blocks - (total_used >> bits);
-        buf->f_bavail = buf->f_blocks - (total_used_data >> bits);
         buf->f_bsize = dentry->d_sb->s_blocksize;
         buf->f_type = BTRFS_SUPER_MAGIC;
+        buf->f_bavail = total_free_data;
+        ret = btrfs_calc_avail_data_space(root, &total_free_data);
+        if (ret) {
+                mutex_unlock(&root->fs_info->chunk_mutex);
+                return ret;
+        }
+        buf->f_bavail += total_free_data;
+        buf->f_bavail = buf->f_bavail >> bits;
+        mutex_unlock(&root->fs_info->chunk_mutex);
 
         /* We treat it as constant endianness (it doesn't matter _which_)
            because we want the fsid to come out the same whether mounted