Btrfs: allow balance to explicitly allocate chunks as it relocates

Btrfs device shrinking and balancing ends up reallocating all the blocks
in order to allow COW to move them to new destinations.  It is somewhat
awkward in terms of ENOSPC because most of the enospc code is built
around the idea that some operation on a reference counted tree triggers
allocations in the non-reference counted trees.

This commit changes the balancing code to deal with enospc by trying to
allocate a new chunk.  If that allocation succeeds, we go ahead and
retry whatever failed due to enospc.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 0825e4ed9447..31ade5802ae8 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3654,6 +3654,7 @@ static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
         u32 item_size;
         int ret;
         int err = 0;
+        int progress = 0;
 
         path = btrfs_alloc_path();
         if (!path)
@@ -3666,9 +3667,10 @@ static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
         }
 
         while (1) {
+                progress++;
                 trans = btrfs_start_transaction(rc->extent_root, 0);
                 BUG_ON(IS_ERR(trans));
-
+restart:
                 if (update_backref_cache(trans, &rc->backref_cache)) {
                         btrfs_end_transaction(trans, rc->extent_root);
                         continue;
@@ -3781,6 +3783,15 @@ static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
                         }
                 }
         }
+        if (trans && progress && err == -ENOSPC) {
+                ret = btrfs_force_chunk_alloc(trans, rc->extent_root,
+                                              rc->block_group->flags);
+                if (ret == 0) {
+                        err = 0;
+                        progress = 0;
+                        goto restart;
+                }
+        }
 
         btrfs_release_path(rc->extent_root, path);
         clear_extent_bits(&rc->processed_blocks, 0, (u64)-1, EXTENT_DIRTY,