btrfs: Don't trim returned range based on input value in find_first_clear_extent_bit

Currently find_first_clear_extent_bit always returns a range whose
starting value is >= passed 'start'. This implicit trimming behavior is
somewhat subtle and an implementation detail.

Instead, this patch modifies the function such that now it always
returns the range which contains passed 'start' and has the given bits
unset. This range could either be due to presence of existing records
which contains 'start' but have the bits unset or because there are no
records that contain the given starting offset.

This patch also adds test cases which cover find_first_clear_extent_bit
since they were missing up until now.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 46 insertions, 6 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 9591ecfaa57e..105b3d2e86f2 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1537,8 +1537,8 @@ out:
 }
 
 /**
- * find_first_clear_extent_bit - finds the first range that has @bits not set
- * and that starts after @start
+ * find_first_clear_extent_bit - find the first range that has @bits not set.
+ * This range could start before @start.
  *
  * @tree - the tree to search
  * @start - the offset at/after which the found extent should start
@@ -1578,12 +1578,52 @@ void find_first_clear_extent_bit(struct extent_io_tree *tree, u64 start,
 				goto out;
 			}
 		}
+		/*
+		 * At this point 'node' either contains 'start' or start is
+		 * before 'node'
+		 */
 		state = rb_entry(node, struct extent_state, rb_node);
-		if (in_range(start, state->start, state->end - state->start + 1) &&
-			(state->state & bits)) {
-			start = state->end + 1;
+
+		if (in_range(start, state->start, state->end - state->start + 1)) {
+			if (state->state & bits) {
+				/*
+				 * |--range with bits sets--|
+				 *    |
+				 *    start
+				 */
+				start = state->end + 1;
+			} else {
+				/*
+				 * 'start' falls within a range that doesn't
+				 * have the bits set, so take its start as
+				 * the beginning of the desired range
+				 *
+				 * |--range with bits cleared----|
+				 *      |
+				 *      start
+				 */
+				*start_ret = state->start;
+				break;
+			}
 		} else {
-			*start_ret = start;
+			/*
+			 * |---prev range---|---hole/unset---|---node range---|
+			 *                          |
+			 *                        start
+			 *
+			 *                        or
+			 *
+			 * |---hole/unset--||--first node--|
+			 * 0   |
+			 *    start
+			 */
+			if (prev) {
+				state = rb_entry(prev, struct extent_state,
+						 rb_node);
+				*start_ret = state->end + 1;
+			} else {
+				*start_ret = 0;
+			}
 			break;
 		}
 	}