btrfs: avoid logging tree mod log elements for irrelevant extent buffers

We are logging tree mod log operations for extent buffers from any tree
but we only need to log for the extent tree and subvolume tree, since
the tree mod log is used to get a consistent view, within a transaction,
of extents and their backrefs. So it's pointless to log operations for
trees such as the csum tree, free space tree, root tree, chunk tree,
log trees, data relocation tree, etc, as these trees are not used for
backref walking and all tree mod log users are about backref walking.

So skip extent buffers that don't belong neither to the extent nor to
subvolume trees. This avoids unnecessary memory allocations and having a
larger tree mod log rbtree with nodes that are never needed.

Reviewed-by: Boris Burkov <[e-mail]>
Signed-off-by: Filipe Manana <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/tree-mod-log.c

···
 	return 0;
 }
 
+static inline bool skip_eb_logging(const struct extent_buffer *eb)
+{
+	const u64 owner = btrfs_header_owner(eb);
+
+	if (btrfs_header_level(eb) == 0)
+		return true;
+
+	/*
+	 * Tree mod logging exists so that there's a consistent view of the
+	 * extents and backrefs of inodes even if while a task is iterating over
+	 * them other tasks are modifying subvolume trees and the extent tree
+	 * (including running delayed refs). So we only need to log extent
+	 * buffers from the extent tree and subvolume trees.
+	 */
+
+	if (owner == BTRFS_EXTENT_TREE_OBJECTID)
+		return false;
+
+	if (btrfs_is_fstree(owner))
+		return false;
+
+	return true;
+}
+
 /*
  * Determines if logging can be omitted. Returns true if it can. Otherwise, it
  * returns false with the tree_mod_log_lock acquired. The caller must hold
···
 {
 	if (!test_bit(BTRFS_FS_TREE_MOD_LOG_USERS, &fs_info->flags))
 		return true;
-	if (eb && btrfs_header_level(eb) == 0)
+	if (eb && skip_eb_logging(eb))
 		return true;
 
 	write_lock(&fs_info->tree_mod_log_lock);
···
 {
 	if (!test_bit(BTRFS_FS_TREE_MOD_LOG_USERS, &fs_info->flags))
 		return false;
-	if (eb && btrfs_header_level(eb) == 0)
+	if (eb && skip_eb_logging(eb))
 		return false;
 
 	return true;