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>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index b40f3cd48961..35c2f8748668 100644
--- a/mm/shmem.c
+++ b/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;