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.
> >
> > 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.
Thanks for analysis! Does the attached patch fix the warning for you?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
>From 5b0cfc37b45670a35228c96cbaee2b99cd3d447c Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@xxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
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
