erofs: fix lockdep false positives on initializing erofs_pseudo_mnt

Lockdep reported the following issue when mounting erofs with a domain_id:

============================================
WARNING: possible recursive locking detected
6.8.0-rc7-xfstests torvalds#521 Not tainted
--------------------------------------------
mount/396 is trying to acquire lock:
ffff907a8aaaa0e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
						at: alloc_super+0xe3/0x3d0

but task is already holding lock:
ffff907a8aaa90e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
						at: alloc_super+0xe3/0x3d0

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&type->s_umount_key#50/1);
  lock(&type->s_umount_key#50/1);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by mount/396:
 #0: ffff907a8aaa90e0 (&type->s_umount_key#50/1){+.+.}-{3:3},
			at: alloc_super+0xe3/0x3d0
 #1: ffffffffc00e6f28 (erofs_domain_list_lock){+.+.}-{3:3},
			at: erofs_fscache_register_fs+0x3d/0x270 [erofs]

stack backtrace:
CPU: 1 PID: 396 Comm: mount Not tainted 6.8.0-rc7-xfstests torvalds#521
Call Trace:
 <TASK>
 dump_stack_lvl+0x64/0xb0
 validate_chain+0x5c4/0xa00
 __lock_acquire+0x6a9/0xd50
 lock_acquire+0xcd/0x2b0
 down_write_nested+0x45/0xd0
 alloc_super+0xe3/0x3d0
 sget_fc+0x62/0x2f0
 vfs_get_super+0x21/0x90
 vfs_get_tree+0x2c/0xf0
 fc_mount+0x12/0x40
 vfs_kern_mount.part.0+0x75/0x90
 kern_mount+0x24/0x40
 erofs_fscache_register_fs+0x1ef/0x270 [erofs]
 erofs_fc_fill_super+0x213/0x380 [erofs]

This is because the file_system_type of both erofs and the pseudo-mount
point of domain_id is erofs_fs_type, so two successive calls to
alloc_super() are considered to be using the same lock and trigger the
warning above.

Therefore add a nodev file_system_type called erofs_anon_fs_type in
fscache.c to silence this complaint. Because kern_mount() takes a
pointer to struct file_system_type, not its (string) name. So we don't
need to call register_filesystem(). In addition, call init_pseudo() in
erofs_anon_init_fs_context() as suggested by Al Viro, so that we can
remove erofs_fc_fill_pseudo_super(), erofs_fc_anon_get_tree(), and
erofs_anon_context_ops.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Fixes: a984956 ("erofs: introduce a pseudo mnt to manage shared cookies")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-and-tested-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Link: https://lore.kernel.org/r/20240307101018.2021925-1-libaokun1@huawei.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Author: LiBaokun96

fs/erofs/fscache.c

@@ -3,6 +3,7 @@
  * Copyright (C) 2022, Alibaba Cloud
  * Copyright (C) 2022, Bytedance Inc. All rights reserved.
  */
+#include <linux/pseudo_fs.h>
 #include <linux/fscache.h>
 #include "internal.h"
 
@@ -12,6 +13,18 @@ static LIST_HEAD(erofs_domain_list);
 static LIST_HEAD(erofs_domain_cookies_list);
 static struct vfsmount *erofs_pseudo_mnt;
 
+static int erofs_anon_init_fs_context(struct fs_context *fc)
+{
+	return init_pseudo(fc, EROFS_SUPER_MAGIC) ? 0 : -ENOMEM;
+}
+
+static struct file_system_type erofs_anon_fs_type = {
+	.owner		= THIS_MODULE,
+	.name           = "pseudo_erofs",
+	.init_fs_context = erofs_anon_init_fs_context,
+	.kill_sb        = kill_anon_super,
+};
+
 struct erofs_fscache_request {
 	struct erofs_fscache_request *primary;
 	struct netfs_cache_resources cache_resources;
@@ -381,7 +394,7 @@ static int erofs_fscache_init_domain(struct super_block *sb)
 		goto out;
 
 	if (!erofs_pseudo_mnt) {
-		struct vfsmount *mnt = kern_mount(&erofs_fs_type);
+		struct vfsmount *mnt = kern_mount(&erofs_anon_fs_type);
 		if (IS_ERR(mnt)) {
 			err = PTR_ERR(mnt);
 			goto out;

fs/erofs/internal.h

@@ -385,7 +385,6 @@ struct erofs_map_dev {
 	unsigned int m_deviceid;
 };
 
-extern struct file_system_type erofs_fs_type;
 extern const struct super_operations erofs_sops;
 
 extern const struct address_space_operations erofs_raw_access_aops;

fs/erofs/super.c

@@ -579,13 +579,6 @@ static const struct export_operations erofs_export_ops = {
 	.get_parent = erofs_get_parent,
 };
 
-static int erofs_fc_fill_pseudo_super(struct super_block *sb, struct fs_context *fc)
-{
-	static const struct tree_descr empty_descr = {""};
-
-	return simple_fill_super(sb, EROFS_SUPER_MAGIC, &empty_descr);
-}
-
 static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	struct inode *inode;
@@ -712,11 +705,6 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 	return 0;
 }
 
-static int erofs_fc_anon_get_tree(struct fs_context *fc)
-{
-	return get_tree_nodev(fc, erofs_fc_fill_pseudo_super);
-}
-
 static int erofs_fc_get_tree(struct fs_context *fc)
 {
 	struct erofs_fs_context *ctx = fc->fs_private;
@@ -789,20 +777,10 @@ static const struct fs_context_operations erofs_context_ops = {
 	.free		= erofs_fc_free,
 };
 
-static const struct fs_context_operations erofs_anon_context_ops = {
-	.get_tree       = erofs_fc_anon_get_tree,
-};
-
 static int erofs_init_fs_context(struct fs_context *fc)
 {
 	struct erofs_fs_context *ctx;
 
-	/* pseudo mount for anon inodes */
-	if (fc->sb_flags & SB_KERNMOUNT) {
-		fc->ops = &erofs_anon_context_ops;
-		return 0;
-	}
-
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 	if (!ctx)
 		return -ENOMEM;
@@ -824,12 +802,6 @@ static void erofs_kill_sb(struct super_block *sb)
 {
 	struct erofs_sb_info *sbi;
 
-	/* pseudo mount for anon inodes */
-	if (sb->s_flags & SB_KERNMOUNT) {
-		kill_anon_super(sb);
-		return;
-	}
-
 	if (erofs_is_fscache_mode(sb))
 		kill_anon_super(sb);
 	else
@@ -868,7 +840,7 @@ static void erofs_put_super(struct super_block *sb)
 	erofs_fscache_unregister_fs(sb);
 }
 
-struct file_system_type erofs_fs_type = {
+static struct file_system_type erofs_fs_type = {
 	.owner          = THIS_MODULE,
 	.name           = "erofs",
 	.init_fs_context = erofs_init_fs_context,