xattr: add rhashtable-based simple_xattr infrastructure

Add rhashtable support to the simple_xattr subsystem while keeping the
existing rbtree code fully functional. This allows consumers to be
migrated one at a time without breaking any intermediate build.

struct simple_xattrs gains a dispatch flag and a union holding either
the rbtree (rb_root + rwlock) or rhashtable state:

  struct simple_xattrs {
      bool use_rhashtable;
      union {
          struct { struct rb_root rb_root; rwlock_t lock; };
          struct rhashtable ht;
      };
  };

simple_xattrs_init() continues to set up the rbtree path for existing
embedded-struct callers.

Add simple_xattrs_alloc() which dynamically allocates a simple_xattrs
and initializes the rhashtable path. This is the entry point for
consumers switching to pointer-based lazy allocation.

The five core functions (get, set, list, add, free) dispatch based on
the use_rhashtable flag.

Existing callers continue to use the rbtree path unchanged. As each
consumer is converted it will switch to simple_xattrs_alloc() and the
rhashtable path. Once all consumers are converted a follow-up patch
will remove the rbtree code.

Link: https://patch.msgid.link/20260216-work-xattr-socket-v1-2-c2efa4f74cb7@kernel.org
Acked-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/xattr.h

@@ -107,8 +107,14 @@ static inline const char *xattr_prefix(const struct xattr_handler *handler)
 }
 
 struct simple_xattrs {
-	struct rb_root rb_root;
-	rwlock_t lock;
+	bool use_rhashtable;
+	union {
+		struct {
+			struct rb_root rb_root;
+			rwlock_t lock;
+		};
+		struct rhashtable ht;
+	};
 };
 
 struct simple_xattr {
@@ -121,6 +127,9 @@ struct simple_xattr {
 };
 
 void simple_xattrs_init(struct simple_xattrs *xattrs);
+struct simple_xattrs *simple_xattrs_alloc(void);
+struct simple_xattrs *simple_xattrs_lazy_alloc(struct simple_xattrs **xattrsp,
+					       const void *value, int flags);
 void simple_xattrs_free(struct simple_xattrs *xattrs, size_t *freed_space);
 size_t simple_xattr_space(const char *name, size_t size);
 struct simple_xattr *simple_xattr_alloc(const void *value, size_t size);
@@ -137,4 +146,16 @@ void simple_xattr_add(struct simple_xattrs *xattrs,
 		      struct simple_xattr *new_xattr);
 int xattr_list_one(char **buffer, ssize_t *remaining_size, const char *name);
 
+DEFINE_CLASS(simple_xattr,
+	     struct simple_xattr *,
+	     if (!IS_ERR_OR_NULL(_T)) simple_xattr_free(_T),
+	     simple_xattr_alloc(value, size),
+	     const void *value, size_t size)
+
+DEFINE_CLASS(simple_xattrs,
+            struct simple_xattrs *,
+            if (!IS_ERR_OR_NULL(_T)) { simple_xattrs_free(_T, NULL); kfree(_T); },
+            simple_xattrs_alloc(),
+            void)
+
 #endif	/* _LINUX_XATTR_H */

mm/shmem.c

@@ -4278,7 +4278,7 @@ static int shmem_initxattrs(struct inode *inode,
 
 	for (xattr = xattr_array; xattr->name != NULL; xattr++) {
 		new_xattr = simple_xattr_alloc(xattr->value, xattr->value_len);
-		if (!new_xattr)
+		if (IS_ERR(new_xattr))
 			break;
 
 		len = strlen(xattr->name) + 1;