Merge patch series "fs/super.c: introduce get_tree_bdev_flags()"

Allison Karlitskaya <[email protected]> says:

In context of my work on composefs/bootc I've been testing the new support
for directly mounting files with erofs (ie: without a loopback device) and
it's working well.  Thanks for adding this feature --- it's a huge quality
of life improvement for us.

I've observed a strange behaviour, though: when mounting a file as an
erofs, if you read() the filesystem context fd, you always get the
following error message reported: Can't lookup blockdev.

That's caused by the code in erofs_fc_get_tree() trying to call
get_tree_bdev() and recovering from the error in case it was ENOTBLK and
CONFIG_EROFS_FS_BACKED_BY_FILE.  Unfortunately, get_tree_bdev() logs the
error directly on the fs_context, so you get the error message even on
successful mounts.

It looks something like this at the syscall level:

fsopen("erofs", FSOPEN_CLOEXEC)         = 3
fsconfig(3, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0
fsconfig(3, FSCONFIG_SET_STRING, "source", "/home/lis/src/mountcfs/cfs", 0)
= 0
fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
fsmount(3, FSMOUNT_CLOEXEC, 0)          = 5
move_mount(5, "", AT_FDCWD, "/tmp/composefs.upper.KuT5aV",
MOVE_MOUNT_F_EMPTY_PATH) = 0
read(3, "e /home/lis/src/mountcfs/cfs: Can't lookup blockdev\n", 1024) = 52

This is kernel 6.12.0-0.rc0.20240926git11a299a7933e.13.fc42.x86_64 from
Fedora Rawhide.

It's a pretty minor issue, but it sent me on a wild goose chase for an hour
or two, so probably it should get fixed before the final release.

Gao Xiang <[email protected]>:

Fix this by providing a get_tree_bdev_flags() helper which can be used
to silence such warnings.

* patches from https://lore.kernel.org/r/[email protected]:
  erofs: use get_tree_bdev_flags() to avoid misleading messages
  fs/super.c: introduce get_tree_bdev_flags()

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

fs/erofs/super.c

@@ -709,7 +709,9 @@ static int erofs_fc_get_tree(struct fs_context *fc)
 	if (IS_ENABLED(CONFIG_EROFS_FS_ONDEMAND) && sbi->fsid)
 		return get_tree_nodev(fc, erofs_fc_fill_super);
 
-	ret = get_tree_bdev(fc, erofs_fc_fill_super);
+	ret = get_tree_bdev_flags(fc, erofs_fc_fill_super,
+		IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE) ?
+			GET_TREE_BDEV_QUIET_LOOKUP : 0);
 #ifdef CONFIG_EROFS_FS_BACKED_BY_FILE
 	if (ret == -ENOTBLK) {
 		if (!fc->source)

fs/super.c

@@ -1596,13 +1596,14 @@ int setup_bdev_super(struct super_block *sb, int sb_flags,
 EXPORT_SYMBOL_GPL(setup_bdev_super);
 
 /**
- * get_tree_bdev - Get a superblock based on a single block device
+ * get_tree_bdev_flags - Get a superblock based on a single block device
  * @fc: The filesystem context holding the parameters
  * @fill_super: Helper to initialise a new superblock
+ * @flags: GET_TREE_BDEV_* flags
  */
-int get_tree_bdev(struct fs_context *fc,
-		int (*fill_super)(struct super_block *,
-				  struct fs_context *))
+int get_tree_bdev_flags(struct fs_context *fc,
+		int (*fill_super)(struct super_block *sb,
+				  struct fs_context *fc), unsigned int flags)
 {
 	struct super_block *s;
 	int error = 0;
@@ -1613,10 +1614,10 @@ int get_tree_bdev(struct fs_context *fc,
 
 	error = lookup_bdev(fc->source, &dev);
 	if (error) {
-		errorf(fc, "%s: Can't lookup blockdev", fc->source);
+		if (!(flags & GET_TREE_BDEV_QUIET_LOOKUP))
+			errorf(fc, "%s: Can't lookup blockdev", fc->source);
 		return error;
 	}
-
 	fc->sb_flags |= SB_NOSEC;
 	s = sget_dev(fc, dev);
 	if (IS_ERR(s))
@@ -1644,6 +1645,19 @@ int get_tree_bdev(struct fs_context *fc,
 	fc->root = dget(s->s_root);
 	return 0;
 }
+EXPORT_SYMBOL_GPL(get_tree_bdev_flags);
+
+/**
+ * get_tree_bdev - Get a superblock based on a single block device
+ * @fc: The filesystem context holding the parameters
+ * @fill_super: Helper to initialise a new superblock
+ */
+int get_tree_bdev(struct fs_context *fc,
+		int (*fill_super)(struct super_block *,
+				  struct fs_context *))
+{
+	return get_tree_bdev_flags(fc, fill_super, 0);
+}
 EXPORT_SYMBOL(get_tree_bdev);
 
 static int test_bdev_super(struct super_block *s, void *data)

include/linux/fs_context.h

@@ -160,6 +160,12 @@ extern int get_tree_keyed(struct fs_context *fc,
 
 int setup_bdev_super(struct super_block *sb, int sb_flags,
 		struct fs_context *fc);
+
+#define GET_TREE_BDEV_QUIET_LOOKUP		0x0001
+int get_tree_bdev_flags(struct fs_context *fc,
+		int (*fill_super)(struct super_block *sb,
+				  struct fs_context *fc), unsigned int flags);
+
 extern int get_tree_bdev(struct fs_context *fc,
 			       int (*fill_super)(struct super_block *sb,
 						 struct fs_context *fc));