[PATCH v3] Improve readability of copy_tree

[Jemmy <jemmywong512@gmail.com>]

by employing `copy mount tree from src to dst` concept.
This involves renaming the opaque variables (e.g., p, q, r, s)
to be more descriptive, aiming to make the code easier to understand.

Changes:
mnt     -> src_root (root of the tree to copy)
r       -> src_child (direct child of the root being cloning)
p       -> src_parent (parent of src_mnt)
s       -> src_mnt (current mount being copying)
parent  -> dst_parent (parent of dst_child)
q       -> dst_mnt (freshly cloned mount)

Signed-off-by: Jemmy <jemmywong512@gmail.com>
---
 fs/namespace.c | 59 ++++++++++++++++++++++++++------------------------
 1 file changed, 31 insertions(+), 28 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 5a51315c6678..0dd43633607b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1966,69 +1966,72 @@ static bool mnt_ns_loop(struct dentry *dentry)
 	return current->nsproxy->mnt_ns->seq >= mnt_ns->seq;
 }
 
-struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
+struct mount *copy_tree(struct mount *src_root, struct dentry *dentry,
 					int flag)
 {
-	struct mount *res, *p, *q, *r, *parent;
+	struct mount *res, *src_parent, *src_child, *src_mnt,
+		*dst_parent, *dst_mnt;
 
-	if (!(flag & CL_COPY_UNBINDABLE) && IS_MNT_UNBINDABLE(mnt))
+	if (!(flag & CL_COPY_UNBINDABLE) && IS_MNT_UNBINDABLE(src_root))
 		return ERR_PTR(-EINVAL);
 
 	if (!(flag & CL_COPY_MNT_NS_FILE) && is_mnt_ns_file(dentry))
 		return ERR_PTR(-EINVAL);
 
-	res = q = clone_mnt(mnt, dentry, flag);
-	if (IS_ERR(q))
-		return q;
+	res = dst_mnt = clone_mnt(src_root, dentry, flag);
+	if (IS_ERR(dst_mnt))
+		return dst_mnt;
 
-	q->mnt_mountpoint = mnt->mnt_mountpoint;
+	src_parent = src_root;
+	dst_mnt->mnt_mountpoint = src_root->mnt_mountpoint;
 
-	p = mnt;
-	list_for_each_entry(r, &mnt->mnt_mounts, mnt_child) {
-		struct mount *s;
-		if (!is_subdir(r->mnt_mountpoint, dentry))
+	list_for_each_entry(src_child, &src_root->mnt_mounts, mnt_child) {
+		if (!is_subdir(src_child->mnt_mountpoint, dentry))
 			continue;
 
-		for (s = r; s; s = next_mnt(s, r)) {
+		for (src_mnt = src_child; src_mnt;
+			src_mnt = next_mnt(src_mnt, src_child)) {
 			if (!(flag & CL_COPY_UNBINDABLE) &&
-			    IS_MNT_UNBINDABLE(s)) {
-				if (s->mnt.mnt_flags & MNT_LOCKED) {
+			    IS_MNT_UNBINDABLE(src_mnt)) {
+				if (src_mnt->mnt.mnt_flags & MNT_LOCKED) {
 					/* Both unbindable and locked. */
-					q = ERR_PTR(-EPERM);
+					dst_mnt = ERR_PTR(-EPERM);
 					goto out;
 				} else {
-					s = skip_mnt_tree(s);
+					src_mnt = skip_mnt_tree(src_mnt);
 					continue;
 				}
 			}
 			if (!(flag & CL_COPY_MNT_NS_FILE) &&
-			    is_mnt_ns_file(s->mnt.mnt_root)) {
-				s = skip_mnt_tree(s);
+			    is_mnt_ns_file(src_mnt->mnt.mnt_root)) {
+				src_mnt = skip_mnt_tree(src_mnt);
 				continue;
 			}
-			while (p != s->mnt_parent) {
-				p = p->mnt_parent;
-				q = q->mnt_parent;
+			while (src_parent != src_mnt->mnt_parent) {
+				src_parent = src_parent->mnt_parent;
+				dst_mnt = dst_mnt->mnt_parent;
 			}
-			p = s;
-			parent = q;
-			q = clone_mnt(p, p->mnt.mnt_root, flag);
-			if (IS_ERR(q))
+
+			src_parent = src_mnt;
+			dst_parent = dst_mnt;
+			dst_mnt = clone_mnt(src_mnt, src_mnt->mnt.mnt_root, flag);
+			if (IS_ERR(dst_mnt))
 				goto out;
 			lock_mount_hash();
-			list_add_tail(&q->mnt_list, &res->mnt_list);
-			attach_mnt(q, parent, p->mnt_mp, false);
+			list_add_tail(&dst_mnt->mnt_list, &res->mnt_list);
+			attach_mnt(dst_mnt, dst_parent, src_parent->mnt_mp, false);
 			unlock_mount_hash();
 		}
 	}
 	return res;
+
 out:
 	if (res) {
 		lock_mount_hash();
 		umount_tree(res, UMOUNT_SYNC);
 		unlock_mount_hash();
 	}
-	return q;
+	return dst_mnt;
 }
 
 /* Caller should check returned pointer for errors */
-- 
2.34.1