From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 15 May 2017 23:34:00 +0300 From: Rakesh Pandit To: Jan Kara , Miklos Szeredi CC: Jens Axboe , , Subject: Re: [PATCH 16/25] fuse: Convert to separately allocated bdi Message-ID: <20170515203400.GA8068@hercules.tuxera.com> References: <20170412102449.16901-1-jack@suse.cz> <20170412102449.16901-17-jack@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" In-Reply-To: <20170412102449.16901-17-jack@suse.cz> List-ID: 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 > CC: linux-fsdevel@vger.kernel.org > Acked-by: Miklos Szeredi > Reviewed-by: Christoph Hellwig > Signed-off-by: Jan Kara ... > 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