btrfs: lift some btrfs_cross_ref_exist checks in nocow path

In nocow path, we check if the extent is snapshotted in
btrfs_cross_ref_exist(). We can do the similar check earlier and avoid
unnecessary search into extent tree.

A fio test on a Intel D-1531, 16GB RAM, SSD RAID-5 machine as follows:

[global]
group_reporting
time_based
thread=1
ioengine=libaio
bs=4k
iodepth=32
size=64G
runtime=180
numjobs=8
rw=randwrite

[file1]
filename=/mnt/nocow/testfile

IOPS result:   unpatched     patched

1 fio round:     46670        46958
snapshot
2 fio round:     51826        54498
3 fio round:     59767        61289

After snapshot, the first fio get about 5% performance gain. As we
continually write to the same file, all writes will resume to nocow mode
and eventually we have no performance gain.

Signed-off-by: Ethan Lien <ethanlien@synology.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update comments ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -1370,6 +1370,13 @@ next_slot:
 			    btrfs_file_extent_encryption(leaf, fi) ||
 			    btrfs_file_extent_other_encoding(leaf, fi))
 				goto out_check;
+			/*
+			 * Do the same check as in btrfs_cross_ref_exist but
+			 * without the unnecessary search.
+			 */
+			if (btrfs_file_extent_generation(leaf, fi) <=
+			    btrfs_root_last_snapshot(&root->root_item))
+				goto out_check;
 			if (extent_type == BTRFS_FILE_EXTENT_REG && !force)
 				goto out_check;
 			if (btrfs_extent_readonly(fs_info, disk_bytenr))
@@ -7322,6 +7329,14 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 	    btrfs_file_extent_other_encoding(leaf, fi))
 		goto out;
 
+	/*
+	 * Do the same check as in btrfs_cross_ref_exist but without the
+	 * unnecessary search.
+	 */
+	if (btrfs_file_extent_generation(leaf, fi) <=
+	    btrfs_root_last_snapshot(&root->root_item))
+		goto out;
+
 	backref_offset = btrfs_file_extent_offset(leaf, fi);
 
 	if (orig_start) {