Re: [Devel] [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

[Stanislav Kinsbursky <skinsbursky@parallels.com>]

11.12.2012 18:56, J. Bruce Fields пишет:
> On Tue, Dec 11, 2012 at 06:12:40PM +0400, Stanislav Kinsbursky wrote:
>> UID: 9899
>>
>> 11.12.2012 18:00, Stanislav Kinsbursky пишет:
>>> 11.12.2012 00:28, J. Bruce Fields пишет:
>>>> On Thu, Dec 06, 2012 at 06:34:47PM +0300, Stanislav Kinsbursky wrote:
>>>>> NFSd does lookup. Lookup is done starting from current->fs->root.
>>>>> NFSd is a kthread, cloned by kthreadd, and thus have global (but luckely
>>>>> unshared) root.
>>>>> So we have to swap root to those, which process, started NFSd, has. Because
>>>>> that process can be in a container with it's own root.
>>>>
>>>> This doesn't sound right to me.
>>>>
>>>> Which lookups exactly do you see being done relative to
>>>> current->fs->root ?
>>>>
>>>
>>> Ok, you are right. I was mistaken here.
>>> This is not a exactly lookup, but d_path() problem in svc_export_request().
>>> I.e. without root swapping, d_path() will give not local export path (like "/export")
>>> but something like this "/root/containers_root/export".
>>>
>>
>> We, actually, can do it less in less aggressive way.
>> I.e. instead root swap and current svc_export_request() implementation:
>>
>> void svc_export_request(...)
>> {
>> 	<snip>
>>          pth = d_path(&exp->ex_path, *bpp, *blen);
>> 	<snip>
>> }
>>
>> we can do something like this:
>>
>> void svc_export_request(...)
>> {
>> 	struct nfsd_net *nn = ...
>> 	<snip>
>> 	spin_lock(&dcache_lock);
>>          pth = __d_path(&exp->ex_path, &nn->root, *bpp, *blen);
>> 	spin_unlock(&dcache_lock);
>> 	<snip>
>> }
>
> That looks simpler, but I still don't understand why we need it.
>
> I'm confused about how d_path works; I would have thought that
> filesystem namespaces would have their own vfsmount trees and hence that
> the (vfsmount, dentry) would be enough to specify the path.  Is the root
> argument for the case of chroot?  Do we care about that?
>

It works very simple: just traverse the tree from specified dentry up to current->fs->root.dentry.
Having container in some fully separated mount point is great, of course. But:
1) this is a limitation we really want to avoid. I.e. container can be chrooted into some path like "/root/containers_root/" as in example above.
2) NFSd kthread works in init root environment. But we anyway want to get proper path string in container root, but not in kthreads root.

> Also, svc_export_request is called from mountd's read of
> /proc/net/rpc/nfsd.export/channel.  If mountd's root is wrong, then
> nothing's going to work anyway.
>

I don't really understand, how  mountd's root can be wrong. I.e. its' always right as I see it. NFSd kthreads have to swap/use relative path/whatever to 
communicate with proper mountd.
Or I'm missing something?

> --b.
>


-- 
Best regards,
Stanislav Kinsbursky