btrfs: allow passing a cached state record to count_range_bits()

An inode's io_tree can be quite large and there are cases where due to
delalloc it can have thousands of extent state records, which makes the
red black tree have a depth of 10 or more, making the operation of
count_range_bits() slow if we repeatedly call it for a range that starts
where, or after, the previous one we called it for. Such use cases are
when searching for delalloc in a file range that corresponds to a hole or
a prealloc extent, which is done during lseek SEEK_HOLE/DATA and fiemap.

So introduce a cached state parameter to count_range_bits() which we use
to store the last extent state record we visited, and then allow the
caller to pass it again on its next call to count_range_bits(). The next
patches in the series will make fiemap and lseek use the new parameter.

This change is part of a patchset that has the goal to make performance
better for applications that use lseek's SEEK_HOLE and SEEK_DATA modes to
iterate over the extents of a file. Two examples are the cp program from
coreutils 9.0+ and the tar program (when using its --sparse / -S option).
A sample test and results are listed in the changelog of the last patch
in the series:

  1/9 btrfs: remove leftover setting of EXTENT_UPTODATE state in an inode's io_tree
  2/9 btrfs: add an early exit when searching for delalloc range for lseek/fiemap
  3/9 btrfs: skip unnecessary delalloc searches during lseek/fiemap
  4/9 btrfs: search for delalloc more efficiently during lseek/fiemap
  5/9 btrfs: remove no longer used btrfs_next_extent_map()
  6/9 btrfs: allow passing a cached state record to count_range_bits()
  7/9 btrfs: update stale comment for count_range_bits()
  8/9 btrfs: use cached state when looking for delalloc ranges with fiemap
  9/9 btrfs: use cached state when looking for delalloc ranges with lseek

Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Link: https://lore.kernel.org/linux-btrfs/20221106073028.71F9.409509F4@e16-tech.com/
Link: https://lore.kernel.org/linux-btrfs/CAL3q7H5NSVicm7nYBJ7x8fFkDpno8z3PYt5aPU43Bajc1H0h1Q@mail.gmail.com/
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-io-tree.c

@@ -1521,9 +1521,11 @@ out:
  */
 u64 count_range_bits(struct extent_io_tree *tree,
 		     u64 *start, u64 search_end, u64 max_bytes,
-		     u32 bits, int contig)
+		     u32 bits, int contig,
+		     struct extent_state **cached_state)
 {
-	struct extent_state *state;
+	struct extent_state *state = NULL;
+	struct extent_state *cached;
 	u64 cur_start = *start;
 	u64 total_bytes = 0;
 	u64 last = 0;
@@ -1534,11 +1536,41 @@ u64 count_range_bits(struct extent_io_tree *tree,
 
 	spin_lock(&tree->lock);
 
+	if (!cached_state || !*cached_state)
+		goto search;
+
+	cached = *cached_state;
+
+	if (!extent_state_in_tree(cached))
+		goto search;
+
+	if (cached->start <= cur_start && cur_start <= cached->end) {
+		state = cached;
+	} else if (cached->start > cur_start) {
+		struct extent_state *prev;
+
+		/*
+		 * The cached state starts after our search range's start. Check
+		 * if the previous state record starts at or before the range we
+		 * are looking for, and if so, use it - this is a common case
+		 * when there are holes between records in the tree. If there is
+		 * no previous state record, we can start from our cached state.
+		 */
+		prev = prev_state(cached);
+		if (!prev)
+			state = cached;
+		else if (prev->start <= cur_start && cur_start <= prev->end)
+			state = prev;
+	}
+
 	/*
 	 * This search will find all the extents that end after our range
 	 * starts.
 	 */
-	state = tree_search(tree, cur_start);
+search:
+	if (!state)
+		state = tree_search(tree, cur_start);
+
 	while (state) {
 		if (state->start > search_end)
 			break;
@@ -1559,7 +1591,16 @@ u64 count_range_bits(struct extent_io_tree *tree,
 		}
 		state = next_state(state);
 	}
+
+	if (cached_state) {
+		free_extent_state(*cached_state);
+		*cached_state = state;
+		if (state)
+			refcount_inc(&state->refs);
+	}
+
 	spin_unlock(&tree->lock);
+
 	return total_bytes;
 }
 

fs/btrfs/extent-io-tree.h

@@ -119,7 +119,8 @@ void __cold extent_state_free_cachep(void);
 
 u64 count_range_bits(struct extent_io_tree *tree,
 		     u64 *start, u64 search_end,
-		     u64 max_bytes, u32 bits, int contig);
+		     u64 max_bytes, u32 bits, int contig,
+		     struct extent_state **cached_state);
 
 void free_extent_state(struct extent_state *state);
 int test_range_bit(struct extent_io_tree *tree, u64 start, u64 end,

fs/btrfs/file.c

@@ -3235,7 +3235,8 @@ static bool find_delalloc_subrange(struct btrfs_inode *inode, u64 start, u64 end
 			*delalloc_start_ret = start;
 			delalloc_len = count_range_bits(&inode->io_tree,
 							delalloc_start_ret, end,
-							len, EXTENT_DELALLOC, 1);
+							len, EXTENT_DELALLOC, 1,
+							NULL);
 		} else {
 			spin_unlock(&inode->lock);
 		}

fs/btrfs/inode.c

@@ -1769,7 +1769,7 @@ static int fallback_to_cow(struct btrfs_inode *inode, struct page *locked_page,
 	 * when starting writeback.
 	 */
 	count = count_range_bits(io_tree, &range_start, end, range_bytes,
-				 EXTENT_NORESERVE, 0);
+				 EXTENT_NORESERVE, 0, NULL);
 	if (count > 0 || is_space_ino || is_reloc_ino) {
 		u64 bytes = count;
 		struct btrfs_fs_info *fs_info = inode->root->fs_info;