vfs: simplify and shrink stack frame of link_path_walk()

Commit 9226b5b4 ("vfs: avoid non-forwarding large load after small
store in path lookup") made link_path_walk() always access the
"hash_len" field as a single 64-bit entity, in order to avoid mixed size
accesses to the members.

However, what I didn't notice was that that effectively means that the
whole "struct qstr this" is now basically redundant.  We already
explicitly track the "const char *name", and if we just use "u64
hash_len" instead of "long len", there is nothing else left of the
"struct qstr".

We do end up wanting the "struct qstr" if we have a filesystem with a
"d_hash()" function, but that's a rare case, and we might as well then
just squirrell away the name and hash_len at that point.

End result: fewer live variables in the loop, a smaller stack frame, and
better code generation.  And we don't need to pass in pointers variables
to helper functions any more, because the return value contains all the
relevant information.  So this removes more lines than it adds, and the
source code is clearer too.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/namei.c

@@ -1674,14 +1674,13 @@ EXPORT_SYMBOL(full_name_hash);
 
 /*
  * Calculate the length and hash of the path component, and
- * fill in the qstr. return the "len" as the result.
+ * return the "hash_len" as the result.
  */
-static inline unsigned long hash_name(const char *name, struct qstr *res)
+static inline u64 hash_name(const char *name)
 {
 	unsigned long a, b, adata, bdata, mask, hash, len;
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 
-	res->name = name;
 	hash = a = 0;
 	len = -sizeof(unsigned long);
 	do {
@@ -1698,9 +1697,7 @@ static inline unsigned long hash_name(const char *name, struct qstr *res)
 
 	hash += a & zero_bytemask(mask);
 	len += find_zero(mask);
-	res->hash_len = hashlen_create(fold_hash(hash), len);
-
-	return len;
+	return hashlen_create(fold_hash(hash), len);
 }
 
 #else
@@ -1718,20 +1715,18 @@ EXPORT_SYMBOL(full_name_hash);
  * We know there's a real path component here of at least
  * one character.
  */
-static inline long hash_name(const char *name, struct qstr *res)
+static inline u64 hash_name(const char *name)
 {
 	unsigned long hash = init_name_hash();
 	unsigned long len = 0, c;
 
-	res->name = name;
 	c = (unsigned char)*name;
 	do {
 		len++;
 		hash = partial_name_hash(c, hash);
 		c = (unsigned char)name[len];
 	} while (c && c != '/');
-	res->hash_len = hashlen_create(end_name_hash(hash), len);
-	return len;
+	return hashlen_create(end_name_hash(hash), len);
 }
 
 #endif
@@ -1756,18 +1751,17 @@ static int link_path_walk(const char *name, struct nameidata *nd)
 
 	/* At this point we know we have a real path component. */
 	for(;;) {
-		struct qstr this;
-		long len;
+		u64 hash_len;
 		int type;
 
 		err = may_lookup(nd);
  		if (err)
 			break;
 
-		len = hash_name(name, &this);
+		hash_len = hash_name(name);
 
 		type = LAST_NORM;
-		if (name[0] == '.') switch (len) {
+		if (name[0] == '.') switch (hashlen_len(hash_len)) {
 			case 2:
 				if (name[1] == '.') {
 					type = LAST_DOTDOT;
@@ -1781,29 +1775,32 @@ static int link_path_walk(const char *name, struct nameidata *nd)
 			struct dentry *parent = nd->path.dentry;
 			nd->flags &= ~LOOKUP_JUMPED;
 			if (unlikely(parent->d_flags & DCACHE_OP_HASH)) {
+				struct qstr this = { .hash_len = hash_len, .name = name };
 				err = parent->d_op->d_hash(parent, &this);
 				if (err < 0)
 					break;
+				hash_len = this.hash_len;
+				name = this.name;
 			}
 		}
 
-		nd->last = this;
+		nd->last.hash_len = hash_len;
+		nd->last.name = name;
 		nd->last_type = type;
 
-		if (!name[len])
+		name += hashlen_len(hash_len);
+		if (!*name)
 			return 0;
 		/*
 		 * If it wasn't NUL, we know it was '/'. Skip that
 		 * slash, and continue until no more slashes.
 		 */
 		do {
-			len++;
-		} while (unlikely(name[len] == '/'));
-		if (!name[len])
+			name++;
+		} while (unlikely(*name == '/'));
+		if (!*name)
 			return 0;
 
-		name += len;
-
 		err = walk_component(nd, &next, LOOKUP_FOLLOW);
 		if (err < 0)
 			return err;