在 2012年2月28日 下午4:41，Miklos Szeredi <miklos@xxxxxxxxxx> 写道：
> Robin Dong <hao.bigrat@xxxxxxxxx> writes:
>
>> Hi, Miklos
>>
>> We are testing and evaluating overlayfs on our machines now. It is
>> stable but we feel unsure about the ovl_link.
>>
>> Imaging using ext4 as upperdir which has a file "hello" and lowdir is
>> totally empty.
>>
>> 1. mount -t overlayfs overlayfs -o lowerdir=/lower,upperdir=/upper /overlay
>> 2. cd /overlay
>> 3. ln hello bye
>>
>> then the overlayfs code will call vfs_link to create a real ext4
>> dentry for "bye" and create
>> a new overlayfs dentry point to overlayfs inode (which standed for
>> "hello"). That means: two
>> overlayfs dentries and only one overlayfs inode.
>>
>> now we continue
>>
>> 4. umount /overlay
>> 5. mount -t overlayfs overlayfs -o lowerdir=/lower,upperdir=/upper
>> /overlay (again)
>> 6. cd /overlay
>> 7. ls hello bye
>>
>> then the overlayfs will create two inodes(one for the "hello", another
>> for the "bye") and two
>> dentries (each point a inode).That means: two dentries and two inodes.
>
> I understand what you are saying but does this have any visible effect?
>
> I mean, is there a way to observe the difference in behavior just from
> userspace or the only difference is in the kernel internal objects?
>
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.
So, we are expecting your suggestions
Thanks.
>> As above, with different order of "create link" and "mount", the
>> result is not the same.
>>
>> My question is: Why not create a new overlayfs inode for "bye" and a
>> new overrlayfs dentry to point it when user do "ln hello bye" ?
>
> Could be done, but I'm interested in a reason for the change.
>
> Thanks,
> Miklos
--
--
Best Regard
Robin Dong
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