[PATCH v3-onstack] btrfs: make block group flags in balance printks human-readable

[Adam Borowski <kilobyte@angband.pl>]

They're not even documented anywhere, letting users with no recourse but
to RTFS.  It's no big burden to output the bitfield as words.

Also, display unknown flags as hex.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
---
 fs/btrfs/relocation.c | 41 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 38 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index c4af0cd..57d867d 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4333,6 +4333,43 @@ static struct reloc_control *alloc_reloc_control(struct btrfs_fs_info *fs_info)
 }
 
 /*
+ * printk the block group being relocated
+ */
+static void describe_relocation(struct btrfs_fs_info *fs_info,
+				struct btrfs_block_group_cache *block_group)
+{
+	char buf[128]; // prefixed by a '|' that'll be dropped
+	u64 flags = block_group->flags;
+
+	if (unlikely(!flags)) // shouldn't happen
+		strcpy(buf, "|NONE");
+	else {
+		char *bp = buf;
+#define DESCRIBE_FLAG(f, d) \
+		if (flags & BTRFS_BLOCK_GROUP_##f) { \
+			bp += snprintf(bp, buf - bp + sizeof(buf), "|%s", d); \
+			flags &= ~BTRFS_BLOCK_GROUP_##f; \
+		}
+		DESCRIBE_FLAG(DATA,     "data");
+		DESCRIBE_FLAG(SYSTEM,   "system");
+		DESCRIBE_FLAG(METADATA, "metadata");
+		DESCRIBE_FLAG(RAID0,    "raid0");
+		DESCRIBE_FLAG(RAID1,    "raid1");
+		DESCRIBE_FLAG(DUP,      "dup");
+		DESCRIBE_FLAG(RAID10,   "raid10");
+		DESCRIBE_FLAG(RAID5,    "raid5");
+		DESCRIBE_FLAG(RAID6,    "raid6");
+		if (unlikely(flags))
+			snprintf(buf, buf - bp + sizeof(buf), "|0x%llx", flags);
+#undef DESCRIBE_FLAG
+	}
+
+	btrfs_info(fs_info,
+		   "relocating block group %llu flags %s",
+		   block_group->key.objectid, buf+1);
+}
+
+/*
  * function to relocate all extents in a block group.
  */
 int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
@@ -4388,9 +4425,7 @@ int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
 		goto out;
 	}
 
-	btrfs_info(extent_root->fs_info,
-		   "relocating block group %llu flags %llu",
-		   rc->block_group->key.objectid, rc->block_group->flags);
+	describe_relocation(extent_root->fs_info, rc->block_group);
 
 	btrfs_wait_block_group_reservations(rc->block_group);
 	btrfs_wait_nocow_writers(rc->block_group);
-- 
2.10.2

This is a version that uses a temp buffer on the stack, but does it in a
separate function so it doesn't cost us anything when deep call chains are
involved.  While balance that can trigger deep call chain, it's not called
deeply itself.

This approach is simpler than mucking with allocs and avoids code
duplication that would be needed for handling failed alloc.