[PATCH 1/2] btrfs-progs: mkfs: Prevent temporary system chunk to use space in reserved 1M range

[Qu Wenruo <wqu@suse.com>]

When creating btrfs, mkfs.btrfs will firstly create a temporary system
chunk as basis, and then created needed trees or new devices.

However the layout temporary system chunk is hard-coded and uses
reserved [0, 1M) range of devid 1.

Change the temporary chunk layout from old:

0	1M				4M	5M
|<----------- temp chunk -------------->|
  And it's 1:1 mapped, which means it's a SINGLE chunk,
  and stripe offset is also 0.

to new layout:

0	1M				4M	5M
	|<----------- temp chunk -------------->|
  And still keeps the 1:1 mapping.

The problem can only be exposed by "-m single" or "-M" where we reuse the
temporary chunk.

With other meta profiles, system and meta chunks are allocated by later
btrfs_alloc_chunk() call, and old SINGLE chunks are removed, so it will
be no such problem for other meta profiles.

Reported-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 mkfs/common.c | 29 +++++++++++++++++++++++------
 mkfs/main.c   |  7 ++++++-
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/mkfs/common.c b/mkfs/common.c
index dd5e7ecff479..5c5e9c3b9e01 100644
--- a/mkfs/common.c
+++ b/mkfs/common.c
@@ -100,6 +100,21 @@ static int btrfs_create_tree_root(int fd, struct btrfs_mkfs_config *cfg,
  *
  * The superblock signature is not valid, denotes a partially created
  * filesystem, needs to be finalized.
+ *
+ * The temporary fs will have the following chunk layout:
+ * Device extent:
+ * 0		1M				5M	......
+ * | Reserved	| dev extent for SYS chunk      |
+ *
+ * And chunk mapping will be:
+ * Chunk mapping:
+ * 0		1M				5M
+ * |		| System chunk, 1:1 mapped	|
+ *
+ * That's to say, there will only be *ONE* system chunk, mapped to
+ * [1M, 5M) physical offset.
+ * And the only chunk is also in logical address [1M, 5M), containing
+ * all essential tree blocks.
  */
 int make_btrfs(int fd, struct btrfs_mkfs_config *cfg)
 {
@@ -154,8 +169,8 @@ int make_btrfs(int fd, struct btrfs_mkfs_config *cfg)
 
 	cfg->blocks[MKFS_SUPER_BLOCK] = BTRFS_SUPER_INFO_OFFSET;
 	for (i = 1; i < MKFS_BLOCK_COUNT; i++) {
-		cfg->blocks[i] = BTRFS_SUPER_INFO_OFFSET + SZ_1M +
-			cfg->nodesize * i;
+		cfg->blocks[i] = BTRFS_BLOCK_RESERVED_1M_FOR_SUPER +
+			cfg->nodesize * (i - 1);
 	}
 
 	btrfs_set_super_bytenr(&super, cfg->blocks[MKFS_SUPER_BLOCK]);
@@ -309,7 +324,7 @@ int make_btrfs(int fd, struct btrfs_mkfs_config *cfg)
 
 	/* then we have chunk 0 */
 	btrfs_set_disk_key_objectid(&disk_key, BTRFS_FIRST_CHUNK_TREE_OBJECTID);
-	btrfs_set_disk_key_offset(&disk_key, 0);
+	btrfs_set_disk_key_offset(&disk_key, BTRFS_BLOCK_RESERVED_1M_FOR_SUPER);
 	btrfs_set_disk_key_type(&disk_key, BTRFS_CHUNK_ITEM_KEY);
 	btrfs_set_item_key(buf, &disk_key, nritems);
 	btrfs_set_item_offset(buf, btrfs_item_nr(nritems), itemoff);
@@ -325,7 +340,8 @@ int make_btrfs(int fd, struct btrfs_mkfs_config *cfg)
 	btrfs_set_chunk_sector_size(buf, chunk, cfg->sectorsize);
 	btrfs_set_chunk_num_stripes(buf, chunk, 1);
 	btrfs_set_stripe_devid_nr(buf, chunk, 0, 1);
-	btrfs_set_stripe_offset_nr(buf, chunk, 0, 0);
+	btrfs_set_stripe_offset_nr(buf, chunk, 0,
+				   BTRFS_BLOCK_RESERVED_1M_FOR_SUPER);
 	nritems++;
 
 	write_extent_buffer(buf, super.dev_item.uuid,
@@ -363,7 +379,7 @@ int make_btrfs(int fd, struct btrfs_mkfs_config *cfg)
 		sizeof(struct btrfs_dev_extent);
 
 	btrfs_set_disk_key_objectid(&disk_key, 1);
-	btrfs_set_disk_key_offset(&disk_key, 0);
+	btrfs_set_disk_key_offset(&disk_key, BTRFS_BLOCK_RESERVED_1M_FOR_SUPER);
 	btrfs_set_disk_key_type(&disk_key, BTRFS_DEV_EXTENT_KEY);
 	btrfs_set_item_key(buf, &disk_key, nritems);
 	btrfs_set_item_offset(buf, btrfs_item_nr(nritems), itemoff);
@@ -374,7 +390,8 @@ int make_btrfs(int fd, struct btrfs_mkfs_config *cfg)
 					BTRFS_CHUNK_TREE_OBJECTID);
 	btrfs_set_dev_extent_chunk_objectid(buf, dev_extent,
 					BTRFS_FIRST_CHUNK_TREE_OBJECTID);
-	btrfs_set_dev_extent_chunk_offset(buf, dev_extent, 0);
+	btrfs_set_dev_extent_chunk_offset(buf, dev_extent,
+					  BTRFS_BLOCK_RESERVED_1M_FOR_SUPER);
 
 	write_extent_buffer(buf, chunk_tree_uuid,
 		    (unsigned long)btrfs_dev_extent_chunk_tree_uuid(dev_extent),
diff --git a/mkfs/main.c b/mkfs/main.c
index d817ad8dfd1a..8e3d19acb6f2 100644
--- a/mkfs/main.c
+++ b/mkfs/main.c
@@ -81,10 +81,15 @@ static int create_metadata_block_groups(struct btrfs_root *root, int mixed,
 	bytes_used = btrfs_super_bytes_used(fs_info->super_copy);
 
 	root->fs_info->system_allocs = 1;
+	/*
+	 * First temporary system chunk must match the chunk layout
+	 * created in make_btrfs().
+	 */
 	ret = btrfs_make_block_group(trans, fs_info, bytes_used,
 				     BTRFS_BLOCK_GROUP_SYSTEM,
 				     BTRFS_FIRST_CHUNK_TREE_OBJECTID,
-				     0, BTRFS_MKFS_SYSTEM_GROUP_SIZE);
+				     BTRFS_BLOCK_RESERVED_1M_FOR_SUPER,
+				     BTRFS_MKFS_SYSTEM_GROUP_SIZE);
 	allocation->system += BTRFS_MKFS_SYSTEM_GROUP_SIZE;
 	if (ret)
 		return ret;
-- 
2.15.1