linux-kernel - Re: [PATCH v3] SUNRPC: set desired file system root before connecting local transports

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Date:	Wed, 10 Oct 2012 09:03:41 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>
Subject: Re: [PATCH v3] SUNRPC: set desired file system root before connecting
 local transports

10.10.2012 02:47, Eric W. Biederman пишет:
> "J. Bruce Fields" <bfields@...ldses.org> writes:
>
>> On Tue, Oct 09, 2012 at 01:20:48PM -0700, Eric W. Biederman wrote:
>>> "Myklebust, Trond" <Trond.Myklebust@...app.com> writes:
>>>
>>>> On Tue, 2012-10-09 at 15:35 -0400, J. Bruce Fields wrote:
>>>>> Cc'ing Eric since I seem to recall he suggested doing it this way?
>>> Yes.  On second look setting fs->root won't work. We need to change fs.
>>> The problem is that by default all kernel threads share fs so changing
>>> fs->root will have non-local consequences.
>> Oh, huh.  And we can't "unshare" it somehow?
> I don't fully understand how nfs uses kernel threads and work queues.
> My general understanding is work queues reuse their kernel threads
> between different users.  So it is mostly a don't pollute your
> environment thing.  If there was a dedicated kernel thread for each
> environment this would be trivial.

One kernel thread per environment is exactly what we are trying to avoid 
if possible.
And the reason why we don't want to do this is that it's really looks 
like overkill.

> What I was suggesting here is changing task->fs instead of
> task->fs.root.  That should just require task_lock().
>
>> Or, previously you suggested:
>>
>> 	- introduce sockaddr_fd that can be applied to AF_UNIX sockets,
>> 	  and teach unix_bind and unix_connect how to deal with a second
>> 	  type of sockaddr, AT_FD:
>> 	  struct sockaddr_fd { short fd_family; short pad; int fd; }
>>
>> 	- introduce sockaddr_unix_at that takes a directory file
>> 	  descriptor as well as a unix path, and teach unix_bind and
>> 	  unix_connect to deal with a second sockaddr type, AF_UNIX_AT:
>> 	  struct sockaddr_unix_at { short family; short pad; int dfd; char path[102]; }
>>
>> Any other options?
> I am still half hoping we don't have to change the userspace API/ABI.
> There is sanity checking on that path that no one seems interested in to
> solve this problem.
>
> This is a weird issue as we are dealing with both the vfs and the
> networking stack.  Fundamentally we need to change task->fs.root or
> we need to capitialize on the openat functionality in the kernel, so
> that we don't create mountains of special cases to support this.
>
> I think swapping task->fs instead of task->fs.root is effecitely the
> same complexity.

Thanks for the idea. And for mentioning, that kernel threads shares fs 
struct. I missed it.

>
>>> I very much believe we want if at all possible to perform a local
>>> modification.
>>>
>>> Changing fs isn't all that different from what devtmpfs is doing.
>> Sorry, I don't know much about devtmpfs, are you suggesting it as a
>> model?  What exactly should we look at?
> Roughly all I meant was that devtmpsfsd is a kernel thread that runs
> with an unshared fs struct.  Although I admit devtmpfsd is for all
> practical purposes a userspace daemon that just happens to run in kernel
> space.
>
> Eric
>
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