On May 16, 2017, at 12:37 PM, Rakesh Pandit <rakesh@xxxxxxxxxx> wrote:
>
>> On Tue, May 16, 2017 at 12:48:31PM +0200, Jan Kara wrote:
>>> On Mon 15-05-17 23:34:00, Rakesh Pandit wrote:
>>> 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.
>>>>
> ....
>>>
>>> ...
>>>
>>>> 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 <rakesh@xxxxxxxxxx>
Jan, want me to add it with the tested-by?
