Robin Dong <hao.bigrat@xxxxxxxxx> writes:
> 在 2012年2月28日 下午4:41，Miklos Szeredi <miklos@xxxxxxxxxx> 写道：
> We have backported the overlayfs to our kernel (2.6.32) and find out
> that when making hard link for a upper file, the kernel will BUG_ON,
> the reason is:
>
> After "ln hello bye" (as the example above and imaging file "hello" is
> an executive program), there are two overlayfs dentries
> point to just one inode. Then we run "./hello", the d_count of "hello"
> dentry become 1 (by __d_lookup), and ovl_permission will try to get
> the alias
> dentry of this inode:
>
> alias = list_entry(inode->i_dentry.next,
> struct dentry, d_alias);
> dget(alias);
>
> but it will find out "bye" dentry and its d_count is zero (because we
> get the inode from another dentry--"hello").
> It will report BUG_ON when trying to dget a zero d_count dentry in
> 2.6.32 kernel:
>
> static inline struct dentry *dget(struct dentry *dentry)
> {
> if (dentry) {
> BUG_ON(!atomic_read(&dentry->d_count));
> atomic_inc(&dentry->d_count);
> }
> return dentry;
> }
>
> There are two ways to fix this problem.
> First, traverse all alias of inode to find a non-zero-d_count dentry,
> this may be inefficient
> Second, create a new overlayfs inode and a new overlayfs dentry in
> ovl_link, that will make the result of two actions
> ("create link" then "mount", or "mount" then "create link")
> consistent.
Okay, I understand the problem now.
Creating a new inode in ovl_link might be the better solution here, I
think.
Thanks,
Miklos
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