Btrfs: keep track of largest extent in bitmaps

We can waste a lot of time searching through bitmaps when we are heavily
fragmented trying to find large contiguous areas that don't exist in the bitmap.
So keep track of the max extent size when we do a full search of a bitmap so
that next time around we can just skip the expensive searching if our max size
is less than what we are looking for.  Thanks,

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

fs/btrfs/free-space-cache.c

@@ -1738,6 +1738,16 @@ static int search_bitmap(struct btrfs_free_space_ctl *ctl,
 	unsigned long next_zero;
 	unsigned long extent_bits;
 
+	/*
+	 * Skip searching the bitmap if we don't have a contiguous section that
+	 * is large enough for this allocation.
+	 */
+	if (bitmap_info->max_extent_size &&
+	    bitmap_info->max_extent_size < *bytes) {
+		*bytes = bitmap_info->max_extent_size;
+		return -1;
+	}
+
 	i = offset_to_bit(bitmap_info->offset, ctl->unit,
 			  max_t(u64, *offset, bitmap_info->offset));
 	bits = bytes_to_bits(*bytes, ctl->unit);
@@ -1762,6 +1772,7 @@ static int search_bitmap(struct btrfs_free_space_ctl *ctl,
 	}
 
 	*bytes = (u64)(max_bits) * ctl->unit;
+	bitmap_info->max_extent_size = *bytes;
 	return -1;
 }
 
@@ -1943,6 +1954,12 @@ static u64 add_bytes_to_bitmap(struct btrfs_free_space_ctl *ctl,
 
 	bitmap_set_bits(ctl, info, offset, bytes_to_set);
 
+	/*
+	 * We set some bytes, we have no idea what the max extent size is
+	 * anymore.
+	 */
+	info->max_extent_size = 0;
+
 	return bytes_to_set;
 
 }
@@ -2782,6 +2799,7 @@ static int btrfs_bitmap_cluster(struct btrfs_block_group_cache *block_group,
 	unsigned long want_bits;
 	unsigned long min_bits;
 	unsigned long found_bits;
+	unsigned long max_bits = 0;
 	unsigned long start = 0;
 	unsigned long total_found = 0;
 	int ret;
@@ -2791,6 +2809,13 @@ static int btrfs_bitmap_cluster(struct btrfs_block_group_cache *block_group,
 	want_bits = bytes_to_bits(bytes, ctl->unit);
 	min_bits = bytes_to_bits(min_bytes, ctl->unit);
 
+	/*
+	 * Don't bother looking for a cluster in this bitmap if it's heavily
+	 * fragmented.
+	 */
+	if (entry->max_extent_size &&
+	    entry->max_extent_size < cont1_bytes)
+		return -ENOSPC;
 again:
 	found_bits = 0;
 	for_each_set_bit_from(i, entry->bitmap, BITS_PER_BITMAP) {
@@ -2798,13 +2823,19 @@ again:
 					       BITS_PER_BITMAP, i);
 		if (next_zero - i >= min_bits) {
 			found_bits = next_zero - i;
+			if (found_bits > max_bits)
+				max_bits = found_bits;
 			break;
 		}
+		if (next_zero - i > max_bits)
+			max_bits = next_zero - i;
 		i = next_zero;
 	}
 
-	if (!found_bits)
+	if (!found_bits) {
+		entry->max_extent_size = (u64)max_bits * ctl->unit;
 		return -ENOSPC;
+	}
 
 	if (!total_found) {
 		start = i;
@@ -3540,6 +3571,7 @@ again:
 		spin_lock(&ctl->tree_lock);
 		info->offset = offset;
 		info->bytes = bytes;
+		info->max_extent_size = 0;
 		ret = link_free_space(ctl, info);
 		spin_unlock(&ctl->tree_lock);
 		if (ret)
@@ -3567,6 +3599,11 @@ again:
 	}
 
 	bytes_added = add_bytes_to_bitmap(ctl, bitmap_info, offset, bytes);
+
+	/* We used the newly allocated info, set the max_extent_size to bytes */
+	if (!info)
+		bitmap_info->max_extent_size = bytes_added;
+
 	bytes -= bytes_added;
 	offset += bytes_added;
 	spin_unlock(&ctl->tree_lock);

fs/btrfs/free-space-cache.h

@@ -23,6 +23,7 @@ struct btrfs_free_space {
 	struct rb_node offset_index;
 	u64 offset;
 	u64 bytes;
+	u64 max_extent_size;
 	unsigned long *bitmap;
 	struct list_head list;
 };