shmem: adapt to rhashtable-based simple_xattrs with lazy allocation

Adapt tmpfs/shmem to use the rhashtable-based xattr path and switch
from an embedded struct to pointer-based lazy allocation.

Change shmem_inode_info.xattrs from embedded 'struct simple_xattrs' to
a pointer 'struct simple_xattrs *', initialized to NULL. This avoids
the rhashtable overhead for every tmpfs inode, which helps when a lot of
inodes exist.

The xattr store is allocated on first use:

- shmem_initxattrs(): Allocates via simple_xattrs_alloc() when
  security modules set initial xattrs during inode creation.

- shmem_xattr_handler_set(): Allocates on first setxattr, with a
  short-circuit for removal when no xattrs are stored yet.

All read paths (shmem_xattr_handler_get, shmem_listxattr) check for
NULL xattrs pointer and return -ENODATA or 0 respectively.

Replaced xattr entries are freed via simple_xattr_free_rcu() to allow
concurrent RCU readers to finish.

shmem_evict_inode() conditionally frees the xattr store only when
allocated.

Also change simple_xattr_add() from void to int to propagate
rhashtable insertion failures. shmem_initxattrs() is the only caller.

Link: https://patch.msgid.link/20260216-work-xattr-socket-v1-3-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>

fs/xattr.c

@@ -1677,19 +1677,19 @@ static bool rbtree_simple_xattr_less(struct rb_node *new_node,
  * of matching xattrs is wanted. Should only be called during inode
  * initialization when a few distinct initial xattrs are supposed to be set.
  */
-void simple_xattr_add(struct simple_xattrs *xattrs,
+int simple_xattr_add(struct simple_xattrs *xattrs,
 		     struct simple_xattr *new_xattr)
 {
-	if (xattrs->use_rhashtable) {
+	if (xattrs->use_rhashtable)
-		WARN_ON(rhashtable_insert_fast(&xattrs->ht,
+		return rhashtable_insert_fast(&xattrs->ht,
 					      &new_xattr->hash_node,
-					       simple_xattr_params));
+					      simple_xattr_params);
-	} else {
+
 	write_lock(&xattrs->lock);
 	rb_add(&new_xattr->rb_node, &xattrs->rb_root,
 	       rbtree_simple_xattr_less);
 	write_unlock(&xattrs->lock);
-	}
+	return 0;
 }
 
 /**

include/linux/shmem_fs.h

@@ -48,7 +48,7 @@ struct shmem_inode_info {
 	};
 	struct timespec64	i_crtime;	/* file creation time */
 	struct shared_policy	policy;		/* NUMA memory alloc policy */
-	struct simple_xattrs	xattrs;		/* list of xattrs */
+	struct simple_xattrs	*xattrs;	/* list of xattrs */
 	pgoff_t			fallocend;	/* highest fallocate endindex */
 	unsigned int		fsflags;	/* for FS_IOC_[SG]ETFLAGS */
 	atomic_t		stop_eviction;	/* hold when working on inode */

include/linux/xattr.h

@@ -142,7 +142,7 @@ struct simple_xattr *simple_xattr_set(struct simple_xattrs *xattrs,
 				      size_t size, int flags);
 ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
 			  char *buffer, size_t size);
-void simple_xattr_add(struct simple_xattrs *xattrs,
+int 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);
 

mm/shmem.c

@@ -1425,7 +1425,10 @@ static void shmem_evict_inode(struct inode *inode)
 		}
 	}
 
-	simple_xattrs_free(&info->xattrs, sbinfo->max_inodes ? &freed : NULL);
+	if (info->xattrs) {
+		simple_xattrs_free(info->xattrs, sbinfo->max_inodes ? &freed : NULL);
+		kfree(info->xattrs);
+	}
 	shmem_free_inode(inode->i_sb, freed);
 	WARN_ON(inode->i_blocks);
 	clear_inode(inode);
@@ -3101,7 +3104,6 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
 		shmem_set_inode_flags(inode, info->fsflags, NULL);
 	INIT_LIST_HEAD(&info->shrinklist);
 	INIT_LIST_HEAD(&info->swaplist);
-	simple_xattrs_init(&info->xattrs);
 	cache_no_acl(inode);
 	if (sbinfo->noswap)
 		mapping_set_unevictable(inode->i_mapping);
@@ -4255,10 +4257,13 @@ static int shmem_initxattrs(struct inode *inode,
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 	const struct xattr *xattr;
-	struct simple_xattr *new_xattr;
 	size_t ispace = 0;
 	size_t len;
 
+	CLASS(simple_xattrs, xattrs)();
+	if (IS_ERR(xattrs))
+		return PTR_ERR(xattrs);
+
 	if (sbinfo->max_inodes) {
 		for (xattr = xattr_array; xattr->name != NULL; xattr++) {
 			ispace += simple_xattr_space(xattr->name,
@@ -4277,24 +4282,24 @@ 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);
+		CLASS(simple_xattr, new_xattr)(xattr->value, xattr->value_len);
 		if (IS_ERR(new_xattr))
 			break;
 
 		len = strlen(xattr->name) + 1;
 		new_xattr->name = kmalloc(XATTR_SECURITY_PREFIX_LEN + len,
 					  GFP_KERNEL_ACCOUNT);
-		if (!new_xattr->name) {
+		if (!new_xattr->name)
-			kvfree(new_xattr);
 			break;
-		}
 
 		memcpy(new_xattr->name, XATTR_SECURITY_PREFIX,
 		       XATTR_SECURITY_PREFIX_LEN);
 		memcpy(new_xattr->name + XATTR_SECURITY_PREFIX_LEN,
 		       xattr->name, len);
 
-		simple_xattr_add(&info->xattrs, new_xattr);
+		if (simple_xattr_add(xattrs, new_xattr))
+			break;
+		retain_and_null_ptr(new_xattr);
 	}
 
 	if (xattr->name != NULL) {
@@ -4303,10 +4308,10 @@ static int shmem_initxattrs(struct inode *inode,
 			sbinfo->free_ispace += ispace;
 			raw_spin_unlock(&sbinfo->stat_lock);
 		}
-		simple_xattrs_free(&info->xattrs, NULL);
 		return -ENOMEM;
 	}
 
+	smp_store_release(&info->xattrs, no_free_ptr(xattrs));
 	return 0;
 }
 
@@ -4315,9 +4320,14 @@ static int shmem_xattr_handler_get(const struct xattr_handler *handler,
 				   const char *name, void *buffer, size_t size)
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct simple_xattrs *xattrs;
+
+	xattrs = READ_ONCE(info->xattrs);
+	if (!xattrs)
+		return -ENODATA;
 
 	name = xattr_full_name(handler, name);
-	return simple_xattr_get(&info->xattrs, name, buffer, size);
+	return simple_xattr_get(xattrs, name, buffer, size);
 }
 
 static int shmem_xattr_handler_set(const struct xattr_handler *handler,
@@ -4328,10 +4338,16 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler,
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
+	struct simple_xattrs *xattrs;
 	struct simple_xattr *old_xattr;
 	size_t ispace = 0;
 
 	name = xattr_full_name(handler, name);
+
+	xattrs = simple_xattrs_lazy_alloc(&info->xattrs, value, flags);
+	if (IS_ERR_OR_NULL(xattrs))
+		return PTR_ERR(xattrs);
+
 	if (value && sbinfo->max_inodes) {
 		ispace = simple_xattr_space(name, size);
 		raw_spin_lock(&sbinfo->stat_lock);
@@ -4344,13 +4360,13 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler,
 			return -ENOSPC;
 	}
 
-	old_xattr = simple_xattr_set(&info->xattrs, name, value, size, flags);
+	old_xattr = simple_xattr_set(xattrs, name, value, size, flags);
 	if (!IS_ERR(old_xattr)) {
 		ispace = 0;
 		if (old_xattr && sbinfo->max_inodes)
 			ispace = simple_xattr_space(old_xattr->name,
 						    old_xattr->size);
-		simple_xattr_free(old_xattr);
+		simple_xattr_free_rcu(old_xattr);
 		old_xattr = NULL;
 		inode_set_ctime_current(inode);
 		inode_inc_iversion(inode);
@@ -4391,7 +4407,9 @@ static const struct xattr_handler * const shmem_xattr_handlers[] = {
 static ssize_t shmem_listxattr(struct dentry *dentry, char *buffer, size_t size)
 {
 	struct shmem_inode_info *info = SHMEM_I(d_inode(dentry));
-	return simple_xattr_list(d_inode(dentry), &info->xattrs, buffer, size);
+
+	return simple_xattr_list(d_inode(dentry), READ_ONCE(info->xattrs),
+				 buffer, size);
 }
 #endif /* CONFIG_TMPFS_XATTR */