Btrfs: fix prealloc under heavy fragmentation conditions

If we are heavily fragmented we will continually try to prealloc the largest
extent size we can every time we call btrfs_reserve_extent.  This can be very
expensive when we are heavily fragmented, burning lots of CPU cycles and loops
through the allocator.  So instead notice when we get a smaller chunk from the
allocator than what we specified and use this as the new maximum size we try to
allocate.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/inode.c

@@ -9687,6 +9687,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 	u64 cur_offset = start;
 	u64 i_size;
 	u64 cur_bytes;
+	u64 last_alloc = (u64)-1;
 	int ret = 0;
 	bool own_trans = true;
 
@@ -9703,6 +9704,13 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 
 		cur_bytes = min(num_bytes, 256ULL * 1024 * 1024);
 		cur_bytes = max(cur_bytes, min_size);
+		/*
+		 * If we are severely fragmented we could end up with really
+		 * small allocations, so if the allocator is returning small
+		 * chunks lets make its job easier by only searching for those
+		 * sized chunks.
+		 */
+		cur_bytes = min(cur_bytes, last_alloc);
 		ret = btrfs_reserve_extent(root, cur_bytes, min_size, 0,
 					   *alloc_hint, &ins, 1, 0);
 		if (ret) {
@@ -9711,6 +9719,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 			break;
 		}
 
+		last_alloc = ins.offset;
 		ret = insert_reserved_file_extent(trans, inode,
 						  cur_offset, ins.objectid,
 						  ins.offset, ins.offset,