[PATCH] btrfs-progs: convert: Mention which reserve_space call failed

[Marcos Paulo de Souza <marcos@mpdesouza.com>]

From: Marcos Paulo de Souza <mpdesouza@suse.com>

btrfs-convert currently can't handle more fragmented block groups when
converting ext4 because the minimum size of a data chunk is 32Mb.

When converting an ext4 fs with more fragmented block group and the disk
almost full, we can end up hitting a ENOSPC problem [1] since smaller
block groups (10Mb for example) end up being extended to 32Mb, leaving
the free space tree smaller when converting it to btrfs.

This patch adds error messages telling which needed bytes couldn't be
allocated from the free space tree:

create btrfs filesystem:
        blocksize: 4096
        nodesize:  16384
        features:  extref, skinny-metadata (default)
        checksum:  crc32c
free space report:
        total:     1073741824
        free:      39124992 (3.64%)
ERROR: failed to reserve 33554432 bytes from free space for metadata chunk
ERROR: unable to create initial ctree: No space left on device

Link: https://github.com/kdave/btrfs-progs/issues/251

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
 convert/common.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/convert/common.c b/convert/common.c
index 048629df..6392e7f4 100644
--- a/convert/common.c
+++ b/convert/common.c
@@ -812,8 +812,10 @@ int make_convert_btrfs(int fd, struct btrfs_mkfs_config *cfg,
 	 */
 	ret = reserve_free_space(free_space, BTRFS_STRIPE_LEN,
 				 &cfg->super_bytenr);
-	if (ret < 0)
+	if (ret < 0) {
+		error("failed to reserve %d bytes from free space for temporary superblock", BTRFS_STRIPE_LEN);
 		goto out;
+	}
 
 	/*
 	 * Then reserve system chunk space
@@ -823,12 +825,16 @@ int make_convert_btrfs(int fd, struct btrfs_mkfs_config *cfg,
 	 */
 	ret = reserve_free_space(free_space, BTRFS_MKFS_SYSTEM_GROUP_SIZE,
 				 &sys_chunk_start);
-	if (ret < 0)
+	if (ret < 0) {
+		error("failed to reserve %d bytes from free space for system chunk", BTRFS_MKFS_SYSTEM_GROUP_SIZE);
 		goto out;
+	}
 	ret = reserve_free_space(free_space, BTRFS_CONVERT_META_GROUP_SIZE,
 				 &meta_chunk_start);
-	if (ret < 0)
+	if (ret < 0) {
+		error("failed to reserve %d bytes from free space for metadata chunk", BTRFS_CONVERT_META_GROUP_SIZE);
 		goto out;
+	}
 
 	/*
 	 * Allocated meta/sys chunks will be mapped 1:1 with device offset.
-- 
2.28.0