Re: 3.1-rc10 oops in nameidata_to_filp

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Nov 24, 2011 at 11:44:06AM -0500, George Spelvin wrote:

> It turned out the machine was quite recoverable and I've been running it without rebooting since then.
> This includes several suspends to RAM and one to disk.
> 
> So far, it seems pretty reproducible, but I suppose it could be a kernel bit flip.
> (F***ing Intel not even *allowing* ECC in "consumer" chipsets...)
> 
> I should probably add a debugging patch and reboot.  Is there a debugging helper
> for printing a dentry and vfsmount?

d_path(); takes struct path *, pointer to buffer and buffer length, puts
the pathname into the end of buffer and returns a pointer to the beginning
of resulting string.

I'd add (hell, maybe start with) printing this:
	file->f_path.dentry->d_inode
	inode
	file->f_mapping
	inode->i_mapping
	inode->i_mapping->host
just to see whether it's open() callback resetting ->f_mapping to NULL or
weird inode->i_mapping->host.  All in case file->f_mapping->host == NULL
just before the spot where it oopses.

Getting pathname would be something like
	static char name[4096];
	struct path path = {.mnt = mnt, .dentry = dentry};
	char *p = d_path(&path, name, 4096);
	if (IS_ERR(p))
		printk("[%d]", PTR_ERR(p));
	else
		printk("'%s'", p);
conditional on the same test.  

Said that, I'm not buying the theory of open assigning to ->f_mapping and
screwing it up; all such assignments end up with ->i_mapping of *some*
inode, as far as I can see from cursory grep over the tree.  Just in case:
do you have CONFIG_FS_POSIX_ACL set?