btrfs: skip unnecessary delalloc searches during lseek/fiemap

During lseek (SEEK_HOLE/DATA) and fiemap, when processing a file range
that corresponds to a hole or a prealloc extent, if we find that there is
no delalloc marked in the inode's io_tree but there is delalloc due to
an extent map in the io tree, then on the next iteration that calls
find_delalloc_subrange() we can skip searching the io tree again, since
on the first call we had no delalloc in the io tree for the whole range.

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/file.c

@@ -3214,6 +3214,7 @@ static long btrfs_fallocate(struct file *file, int mode,
  * looping while it gets adjacent subranges, and merging them together.
  */
 static bool find_delalloc_subrange(struct btrfs_inode *inode, u64 start, u64 end,
+				   bool *search_io_tree,
 				   u64 *delalloc_start_ret, u64 *delalloc_end_ret)
 {
 	u64 len = end + 1 - start;
@@ -3231,7 +3232,7 @@ static bool find_delalloc_subrange(struct btrfs_inode *inode, u64 start, u64 end
 	spin_lock(&inode->lock);
 	outstanding_extents = inode->outstanding_extents;
 
-	if (inode->delalloc_bytes > 0) {
+	if (*search_io_tree && inode->delalloc_bytes > 0) {
 		spin_unlock(&inode->lock);
 		*delalloc_start_ret = start;
 		delalloc_len = count_range_bits(&inode->io_tree,
@@ -3257,6 +3258,9 @@ static bool find_delalloc_subrange(struct btrfs_inode *inode, u64 start, u64 end
 			start = *delalloc_end_ret + 1;
 			len = end + 1 - start;
 		}
+	} else {
+		/* No delalloc, future calls don't need to search again. */
+		*search_io_tree = false;
 	}
 
 	/*
@@ -3390,6 +3394,7 @@ bool btrfs_find_delalloc_in_range(struct btrfs_inode *inode, u64 start, u64 end,
 {
 	u64 cur_offset = round_down(start, inode->root->fs_info->sectorsize);
 	u64 prev_delalloc_end = 0;
+	bool search_io_tree = true;
 	bool ret = false;
 
 	while (cur_offset < end) {
@@ -3398,6 +3403,7 @@ bool btrfs_find_delalloc_in_range(struct btrfs_inode *inode, u64 start, u64 end,
 		bool delalloc;
 
 		delalloc = find_delalloc_subrange(inode, cur_offset, end,
+						  &search_io_tree,
 						  &delalloc_start,
 						  &delalloc_end);
 		if (!delalloc)