Re: [PATCH 16/25] fuse: Convert to separately allocated bdi

[Rakesh Pandit <rakesh@tuxera.com>]

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@tuxera.com>

> From 5b0cfc37b45670a35228c96cbaee2b99cd3d447c Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@suse.cz>
> Date: Tue, 16 May 2017 12:22:22 +0200
> Subject: [PATCH] fuseblk: Fix warning in super_setup_bdi_name()
> 
> Commit 5f7f7543f52e "fuse: Convert to separately allocated bdi" didn't
> properly handle fuseblk filesystem. When fuse_bdi_init() is called for
> that filesystem type, sb->s_bdi is already initialized (by
> set_bdev_super()) to point to block device's bdi and consequently
> super_setup_bdi_name() complains about this fact when reseting bdi to
> the private one.
> 
> Fix the problem by properly dropping bdi reference in fuse_bdi_init()
> before creating a private bdi in super_setup_bdi_name().
> 
> Fixes: 5f7f7543f52eee03ed35c9d671fbb1cdbd4bc9b5
> Reported-by: Rakesh Pandit <rakesh@tuxera.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/fuse/inode.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 5a1b58f8fef4..65c88379a3a1 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -975,8 +975,15 @@ static int fuse_bdi_init(struct fuse_conn *fc, struct super_block *sb)
>  	int err;
>  	char *suffix = "";
>  
> -	if (sb->s_bdev)
> +	if (sb->s_bdev) {
>  		suffix = "-fuseblk";
> +		/*
> +		 * sb->s_bdi points to blkdev's bdi however we want to redirect
> +		 * it to our private bdi...
> +		 */
> +		bdi_put(sb->s_bdi);
> +		sb->s_bdi = &noop_backing_dev_info;
> +	}
>  	err = super_setup_bdi_name(sb, "%u:%u%s", MAJOR(fc->dev),
>  				   MINOR(fc->dev), suffix);
>  	if (err)
> -- 
> 2.12.0
>