Hi Jan, Miklos,
On Wed, Apr 12, 2017 at 12:24:40PM +0200, Jan Kara wrote:
> Allocate struct backing_dev_info separately instead of embedding it
> inside the superblock. This unifies handling of bdi among users.
>
> CC: Miklos Szeredi <miklos@xxxxxxxxxx>
> CC: linux-fsdevel@xxxxxxxxxxxxxxx
> Acked-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
...
> 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.
Best regards,
Rakesh
