btrfs: get the next extent map during fiemap/lseek more efficiently

At find_delalloc_subrange(), when we need to get the next extent map, we
do a full search on the extent map tree (a red black tree). This is fine
but it's a lot more efficient to simply use rb_next(), which typically
requires iterating over less nodes of the tree and never needs to compare
the ranges of nodes with the one we are looking for.

So add a public helper to extent_map.{h,c} to get the extent map that
immediately follows another extent map, using rb_next(), and use that
helper at find_delalloc_subrange().
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -523,7 +523,7 @@ void replace_extent_mapping(struct extent_map_tree *tree,
 	setup_extent_mapping(tree, new, modified);
 }
 
-static struct extent_map *next_extent_map(struct extent_map *em)
+static struct extent_map *next_extent_map(const struct extent_map *em)
 {
 	struct rb_node *next;
 
@@ -533,6 +533,35 @@ static struct extent_map *next_extent_map(struct extent_map *em)
 	return container_of(next, struct extent_map, rb_node);
 }
 
+/*
+ * Get the extent map that immediately follows another one.
+ *
+ * @tree:       The extent map tree that the extent map belong to.
+ *              Holding read or write access on the tree's lock is required.
+ * @em:         An extent map from the given tree. The caller must ensure that
+ *              between getting @em and between calling this function, the
+ *              extent map @em is not removed from the tree - for example, by
+ *              holding the tree's lock for the duration of those 2 operations.
+ *
+ * Returns the extent map that immediately follows @em, or NULL if @em is the
+ * last extent map in the tree.
+ */
+struct extent_map *btrfs_next_extent_map(const struct extent_map_tree *tree,
+					 const struct extent_map *em)
+{
+	struct extent_map *next;
+
+	/* The lock must be acquired either in read mode or write mode. */
+	lockdep_assert_held(&tree->lock);
+	ASSERT(extent_map_in_tree(em));
+
+	next = next_extent_map(em);
+	if (next)
+		refcount_inc(&next->refs);
+
+	return next;
+}
+
 static struct extent_map *prev_extent_map(struct extent_map *em)
 {
 	struct rb_node *prev;

fs/btrfs/extent_map.h

@@ -87,6 +87,8 @@ static inline u64 extent_map_block_end(struct extent_map *em)
 void extent_map_tree_init(struct extent_map_tree *tree);
 struct extent_map *lookup_extent_mapping(struct extent_map_tree *tree,
 					 u64 start, u64 len);
+struct extent_map *btrfs_next_extent_map(const struct extent_map_tree *tree,
+					 const struct extent_map *em);
 int add_extent_mapping(struct extent_map_tree *tree,
 		       struct extent_map *em, int modified);
 void remove_extent_mapping(struct extent_map_tree *tree, struct extent_map *em);

fs/btrfs/file.c

@@ -3569,40 +3569,50 @@ static bool find_delalloc_subrange(struct btrfs_inode *inode, u64 start, u64 end
 	 */
 	read_lock(&em_tree->lock);
 	em = lookup_extent_mapping(em_tree, start, len);
-	read_unlock(&em_tree->lock);
+	if (!em) {
+		read_unlock(&em_tree->lock);
+		return (delalloc_len > 0);
+	}
 
 	/* extent_map_end() returns a non-inclusive end offset. */
-	em_end = em ? extent_map_end(em) : 0;
+	em_end = extent_map_end(em);
 
 	/*
 	 * If we have a hole/prealloc extent map, check the next one if this one
 	 * ends before our range's end.
 	 */
-	if (em && (em->block_start == EXTENT_MAP_HOLE ||
-		   test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) && em_end < end) {
+	if ((em->block_start == EXTENT_MAP_HOLE ||
+	     test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) && em_end < end) {
 		struct extent_map *next_em;
 
-		read_lock(&em_tree->lock);
-		next_em = lookup_extent_mapping(em_tree, em_end, len - em_end);
-		read_unlock(&em_tree->lock);
-
+		next_em = btrfs_next_extent_map(em_tree, em);
 		free_extent_map(em);
-		em_end = next_em ? extent_map_end(next_em) : 0;
+
+		/*
+		 * There's no next extent map or the next one starts beyond our
+		 * range, return the range found in the io tree (if any).
+		 */
+		if (!next_em || next_em->start > end) {
+			read_unlock(&em_tree->lock);
+			free_extent_map(next_em);
+			return (delalloc_len > 0);
+		}
+
+		em_end = extent_map_end(next_em);
 		em = next_em;
 	}
 
-	if (em && (em->block_start == EXTENT_MAP_HOLE ||
-		   test_bit(EXTENT_FLAG_PREALLOC, &em->flags))) {
-		free_extent_map(em);
-		em = NULL;
-	}
+	read_unlock(&em_tree->lock);
 
 	/*
-	 * No extent map or one for a hole or prealloc extent. Use the delalloc
-	 * range we found in the io tree if we have one.
+	 * We have a hole or prealloc extent that ends at or beyond our range's
+	 * end, return the range found in the io tree (if any).
 	 */
-	if (!em)
+	if (em->block_start == EXTENT_MAP_HOLE ||
+	    test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) {
+		free_extent_map(em);
 		return (delalloc_len > 0);
+	}
 
 	/*
 	 * We don't have any range as EXTENT_DELALLOC in the io tree, so the