[PATCH 04/14] kernfs: adapt to rhashtable-based simple_xattrs with lazy allocation

[Christian Brauner <brauner@kernel.org>]

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

Change kernfs_iattrs.xattrs from embedded 'struct simple_xattrs' to a
pointer 'struct simple_xattrs *', initialized to NULL (zeroed by
kmem_cache_zalloc). Since kernfs_iattrs is already lazily allocated
itself, this adds a second level of lazy allocation specifically for
the xattr store.

The xattr store is allocated on first setxattr. Read paths
check for NULL and return -ENODATA or empty list.

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

The cleanup paths in kernfs_free_rcu() and __kernfs_new_node() error
handling conditionally free the xattr store only when allocated.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/kernfs/dir.c             | 15 +++++++++++----
 fs/kernfs/inode.c           | 34 +++++++++++++++++++++++++---------
 fs/kernfs/kernfs-internal.h |  2 +-
 3 files changed, 37 insertions(+), 14 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 29baeeb97871..e5735c45fb99 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -547,10 +547,8 @@ static void kernfs_free_rcu(struct rcu_head *rcu)
 	/* If the whole node goes away, then name can't be used outside */
 	kfree_const(rcu_access_pointer(kn->name));
 
-	if (kn->iattr) {
-		simple_xattrs_free(&kn->iattr->xattrs, NULL);
+	if (kn->iattr)
 		kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
-	}
 
 	kmem_cache_free(kernfs_node_cache, kn);
 }
@@ -584,6 +582,12 @@ void kernfs_put(struct kernfs_node *kn)
 	if (kernfs_type(kn) == KERNFS_LINK)
 		kernfs_put(kn->symlink.target_kn);
 
+	if (kn->iattr && kn->iattr->xattrs) {
+		simple_xattrs_free(kn->iattr->xattrs, NULL);
+		kfree(kn->iattr->xattrs);
+		kn->iattr->xattrs = NULL;
+	}
+
 	spin_lock(&root->kernfs_idr_lock);
 	idr_remove(&root->ino_idr, (u32)kernfs_ino(kn));
 	spin_unlock(&root->kernfs_idr_lock);
@@ -682,7 +686,10 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 
  err_out4:
 	if (kn->iattr) {
-		simple_xattrs_free(&kn->iattr->xattrs, NULL);
+		if (kn->iattr->xattrs) {
+			simple_xattrs_free(kn->iattr->xattrs, NULL);
+			kfree(kn->iattr->xattrs);
+		}
 		kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
 	}
  err_out3:
diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index a36aaee98dce..dfc3315b5afc 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -45,7 +45,6 @@ static struct kernfs_iattrs *__kernfs_iattrs(struct kernfs_node *kn, bool alloc)
 	ret->ia_mtime = ret->ia_atime;
 	ret->ia_ctime = ret->ia_atime;
 
-	simple_xattrs_init(&ret->xattrs);
 	atomic_set(&ret->nr_user_xattrs, 0);
 	atomic_set(&ret->user_xattr_size, 0);
 
@@ -146,7 +145,8 @@ ssize_t kernfs_iop_listxattr(struct dentry *dentry, char *buf, size_t size)
 	if (!attrs)
 		return -ENOMEM;
 
-	return simple_xattr_list(d_inode(dentry), &attrs->xattrs, buf, size);
+	return simple_xattr_list(d_inode(dentry), READ_ONCE(attrs->xattrs),
+				 buf, size);
 }
 
 static inline void set_default_inode_attr(struct inode *inode, umode_t mode)
@@ -298,27 +298,38 @@ int kernfs_xattr_get(struct kernfs_node *kn, const char *name,
 		     void *value, size_t size)
 {
 	struct kernfs_iattrs *attrs = kernfs_iattrs_noalloc(kn);
+	struct simple_xattrs *xattrs;
+
 	if (!attrs)
 		return -ENODATA;
 
-	return simple_xattr_get(&attrs->xattrs, name, value, size);
+	xattrs = READ_ONCE(attrs->xattrs);
+	if (!xattrs)
+		return -ENODATA;
+
+	return simple_xattr_get(xattrs, name, value, size);
 }
 
 int kernfs_xattr_set(struct kernfs_node *kn, const char *name,
 		     const void *value, size_t size, int flags)
 {
 	struct simple_xattr *old_xattr;
+	struct simple_xattrs *xattrs;
 	struct kernfs_iattrs *attrs;
 
 	attrs = kernfs_iattrs(kn);
 	if (!attrs)
 		return -ENOMEM;
 
-	old_xattr = simple_xattr_set(&attrs->xattrs, name, value, size, flags);
+	xattrs = simple_xattrs_lazy_alloc(&attrs->xattrs, value, flags);
+	if (IS_ERR_OR_NULL(xattrs))
+		return PTR_ERR(xattrs);
+
+	old_xattr = simple_xattr_set(xattrs, name, value, size, flags);
 	if (IS_ERR(old_xattr))
 		return PTR_ERR(old_xattr);
 
-	simple_xattr_free(old_xattr);
+	simple_xattr_free_rcu(old_xattr);
 	return 0;
 }
 
@@ -376,7 +387,7 @@ static int kernfs_vfs_user_xattr_add(struct kernfs_node *kn,
 
 	ret = 0;
 	size = old_xattr->size;
-	simple_xattr_free(old_xattr);
+	simple_xattr_free_rcu(old_xattr);
 dec_size_out:
 	atomic_sub(size, sz);
 dec_count_out:
@@ -403,7 +414,7 @@ static int kernfs_vfs_user_xattr_rm(struct kernfs_node *kn,
 
 	atomic_sub(old_xattr->size, sz);
 	atomic_dec(nr);
-	simple_xattr_free(old_xattr);
+	simple_xattr_free_rcu(old_xattr);
 	return 0;
 }
 
@@ -415,6 +426,7 @@ static int kernfs_vfs_user_xattr_set(const struct xattr_handler *handler,
 {
 	const char *full_name = xattr_full_name(handler, suffix);
 	struct kernfs_node *kn = inode->i_private;
+	struct simple_xattrs *xattrs;
 	struct kernfs_iattrs *attrs;
 
 	if (!(kernfs_root(kn)->flags & KERNFS_ROOT_SUPPORT_USER_XATTR))
@@ -424,11 +436,15 @@ static int kernfs_vfs_user_xattr_set(const struct xattr_handler *handler,
 	if (!attrs)
 		return -ENOMEM;
 
+	xattrs = simple_xattrs_lazy_alloc(&attrs->xattrs, value, flags);
+	if (IS_ERR_OR_NULL(xattrs))
+		return PTR_ERR(xattrs);
+
 	if (value)
-		return kernfs_vfs_user_xattr_add(kn, full_name, &attrs->xattrs,
+		return kernfs_vfs_user_xattr_add(kn, full_name, xattrs,
 						 value, size, flags);
 	else
-		return kernfs_vfs_user_xattr_rm(kn, full_name, &attrs->xattrs,
+		return kernfs_vfs_user_xattr_rm(kn, full_name, xattrs,
 						value, size, flags);
 
 }
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 6061b6f70d2a..1324ed8c0661 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -26,7 +26,7 @@ struct kernfs_iattrs {
 	struct timespec64	ia_mtime;
 	struct timespec64	ia_ctime;
 
-	struct simple_xattrs	xattrs;
+	struct simple_xattrs	*xattrs;
 	atomic_t		nr_user_xattrs;
 	atomic_t		user_xattr_size;
 };

-- 
2.47.3