On Tue, May 16, 2017 at 12:48:31PM +0200, Jan Kara wrote:

On Mon 15-05-17 23:34:00, Rakesh Pandit wrote:

quoted

Hi Jan, Miklos,

On Wed, Apr 12, 2017 at 12:24:40PM +0200, Jan Kara wrote:

quoted

Allocate struct backing_dev_info separately instead of embedding it
inside the superblock. This unifies handling of bdi among users.

....

quoted

...

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 static int fuse_bdi_init(struct fuse_conn *fc, struct super_block *sb)
 {
 	int err;
+	char *suffix = "";
 
-	fc->bdi.name = "fuse";
-	fc->bdi.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_SIZE;
-	/* fuse does it's own writeback accounting */
-	fc->bdi.capabilities = BDI_CAP_NO_ACCT_WB | BDI_CAP_STRICTLIMIT;
-
-	err = bdi_init(&fc->bdi);
+	if (sb->s_bdev)
+		suffix = "-fuseblk";
+	err = super_setup_bdi_name(sb, "%u:%u%s", MAJOR(fc->dev),
+				   MINOR(fc->dev), suffix);
 	if (err)
 		return err;

This call to super_setup_bdi_name would only work with "fuse" but not
with "fuseblk" as mounting a block device in userspace triggers
mount_bdev call which results in set_bdev_super taking a reference
from block device's BDI.  But super_setup_bdi_name allocates a new bdi
and ignores the already existing reference which triggers:

WARN_ON(sb->s_bdi != &noop_backing_dev_info);

as sb->s_bdi already has a reference from set_bdev_super.  This works
for "fuse" (without a blocking device) for obvious reasons.  I can
reproduce this on -rc1 and also found a report on lkml:
https://lkml.org/lkml/2017/5/2/445

Only sane solution seems to be maintaining a private bdi instace just
for fuseblk and let fuse use the common new infrastructure.

Thanks for analysis! Does the attached patch fix the warning for you?

Yes, tested. Feel free to add:
Tested-by: Rakesh Pandit <redacted>