+/*
+ * 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;
+}
+