1 files changed, 41 insertions, 6 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index ee6abb8fa71..fef1f212860 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4486,14 +4486,49 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 		 * If the inodes csum_bytes is the same as the original
 		 * csum_bytes then we know we haven't raced with any free()ers
 		 * so we can just reduce our inodes csum bytes and carry on.
-		 * Otherwise we have to do the normal free thing to account for
-		 * the case that the free side didn't free up its reserve
-		 * because of this outstanding reservation.
 		 */
-		if (BTRFS_I(inode)->csum_bytes == csum_bytes)
+		if (BTRFS_I(inode)->csum_bytes == csum_bytes) {
 			calc_csum_metadata_size(inode, num_bytes, 0);
-		else
-			to_free = calc_csum_metadata_size(inode, num_bytes, 0);
+		} else {
+			u64 orig_csum_bytes = BTRFS_I(inode)->csum_bytes;
+			u64 bytes;
+
+			/*
+			 * This is tricky, but first we need to figure out how much we
+			 * free'd from any free-ers that occured during this
+			 * reservation, so we reset ->csum_bytes to the csum_bytes
+			 * before we dropped our lock, and then call the free for the
+			 * number of bytes that were freed while we were trying our
+			 * reservation.
+			 */
+			bytes = csum_bytes - BTRFS_I(inode)->csum_bytes;
+			BTRFS_I(inode)->csum_bytes = csum_bytes;
+			to_free = calc_csum_metadata_size(inode, bytes, 0);
+
+
+			/*
+			 * Now we need to see how much we would have freed had we not
+			 * been making this reservation and our ->csum_bytes were not
+			 * artificially inflated.
+			 */
+			BTRFS_I(inode)->csum_bytes = csum_bytes - num_bytes;
+			bytes = csum_bytes - orig_csum_bytes;
+			bytes = calc_csum_metadata_size(inode, bytes, 0);
+
+			/*
+			 * Now reset ->csum_bytes to what it should be.  If bytes is
+			 * more than to_free then we would have free'd more space had we
+			 * not had an artificially high ->csum_bytes, so we need to free
+			 * the remainder.  If bytes is the same or less then we don't
+			 * need to do anything, the other free-ers did the correct
+			 * thing.
+			 */
+			BTRFS_I(inode)->csum_bytes = orig_csum_bytes - num_bytes;
+			if (bytes > to_free)
+				to_free = bytes - to_free;
+			else
+				to_free = 0;
+		}
 		spin_unlock(&BTRFS_I(inode)->lock);
 		if (dropped)
 			to_free += btrfs_calc_trans_metadata_size(root, dropped);