btrfs: Add chunk allocation ENOSPC debug message for enospc_debug mount option

Enospc_debug makes extent allocator print more debug messages,
however for chunk allocation, there is no debug message for enospc_debug
at all.

This patch will add message for the following parts of chunk allocator:

1) No rw device at all
   Quite rare, but at least output one message for this case.

2) Not enough space for some device
   This debug message is quite handy for unbalanced disks with stripe
   based profiles (RAID0/10/5/6).

3) Not enough free devices
   This debug message should tell us if current chunk allocator is
   working correctly under minimal device requirements.

Although in most cases, we will hit other ENOSPC before we even hit a
chunk allocator ENOSPC, but such debug info won't help.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -4700,8 +4700,11 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 
 	BUG_ON(!alloc_profile_is_valid(type, 0));
 
-	if (list_empty(&fs_devices->alloc_list))
+	if (list_empty(&fs_devices->alloc_list)) {
+		if (btrfs_test_opt(info, ENOSPC_DEBUG))
+			btrfs_debug(info, "%s: no writable device", __func__);
 		return -ENOSPC;
+	}
 
 	index = __get_raid_index(type);
 
@@ -4784,8 +4787,14 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 		if (ret == 0)
 			max_avail = max_stripe_size * dev_stripes;
 
-		if (max_avail < BTRFS_STRIPE_LEN * dev_stripes)
+		if (max_avail < BTRFS_STRIPE_LEN * dev_stripes) {
+			if (btrfs_test_opt(info, ENOSPC_DEBUG))
+				btrfs_debug(info,
+			"%s: devid %llu has no free space, have=%llu want=%u",
+					    __func__, device->devid, max_avail,
+					    BTRFS_STRIPE_LEN * dev_stripes);
 			continue;
+		}
 
 		if (ndevs == fs_devices->rw_devices) {
 			WARN(1, "%s: found more than %llu devices\n",
@@ -4810,6 +4819,12 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 
 	if (ndevs < devs_increment * sub_stripes || ndevs < devs_min) {
 		ret = -ENOSPC;
+		if (btrfs_test_opt(info, ENOSPC_DEBUG)) {
+			btrfs_debug(info,
+	"%s: not enough devices with free space: have=%d minimum required=%d",
+				    __func__, ndevs, min(devs_min,
+				    devs_increment * sub_stripes));
+		}
 		goto error;
 	}