Re: [Jfs-discussion] Kernel oops when accessing to mounted, but unplugged JFS

[Dave Kleikamp <shaggy@linux.vnet.ibm.com>]

On Mon, 2010-11-22 at 23:20 +0200, Alexander Kolesen wrote:

> > I haven't dug too far into this yet, but I suspect that
> > aaead25b954879e1a708ff2f3602f494c18d20b5 is related.
> > 
> > commit aaead25b954879e1a708ff2f3602f494c18d20b5
> > Author: Christoph Hellwig <hch@lst.de>
> > Date:   Mon Oct 4 14:25:33 2010 +0200
> > 
> >     writeback: always use sb->s_bdi for writeback purposes
> > 
> > I'll have to look at what happens when a device is unplugged to see if
> > JFS is missing something, or it's more of a generic problem.  I'm open
> > to suggestions from anybody on cc.
> > 
> > Thanks,
> > Shaggy
> >
> 
> I've tested kernel before and after this commit. Yes, it reproduced after,
> and didn't reproduced before.

I recreated the problem on ext3 as well, so it's not specific to JFS.

I see three potential ways to fix this.

1. bdi_prune_sb() could set sb->s_bdi to &default_backing_dev_info
rather than NULL
2. inode_to_bdi() could return &default_backing_dev_info (or
inode->i_mapping->backing_dev_info) if sb->s_bdi is NULL.
3. the callers of inode_to_bdi() could check for s_bdi being NULL and
exit gracefully.

It seems that Jens and Christoph have ideas about cleaning up the bdi
stuff, so this may be a short-term fix.

Here's a patch for option 2.
  ---------------------------------
fs: avoid null pointer dereference when a block device is unplugged

Physically unplugging a block device when a file system is mounted can
result in sb->s_bdi being set to NULL.  The callers of inode_to_bdi()
expect a non-NULL pointer.  Return &default_backing_dev_info instead
of NULL.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
---
 fs/fs-writeback.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3d06ccc..5f8cc5d 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -76,7 +76,7 @@ static inline struct backing_dev_info *inode_to_bdi(struct inode *inode)
 	if (strcmp(sb->s_type->name, "bdev") == 0)
 		return inode->i_mapping->backing_dev_info;
 
-	return sb->s_bdi;
+	return sb->s_bdi ? sb->s_bdi : &default_backing_dev_info;
 }
 
 static inline struct inode *wb_inode(struct list_head *head)


-- 
Dave Kleikamp
IBM Linux Technology Center