Re: [PATCH] autofs: show pipe inode in mount options

[Stanislav Kinsburskiy]

08.01.2016 08:20, Ian Kent ÐÐÑÐÑ:
> On Thu, 2016-01-07 at 16:46 +0100, Stanislav Kinsburskiy wrote:
> > Good day, gentlemen.
> >
> > Could you update, what's the status with this patch?
> > Without it it's impossible to match process pipe with kernel pipe,
> > while
> > this is "must have" to be able to migrate AutoFS via CRIU.
> Right, I did mean to reply to this mail but have been distracted by
> family stuff.
>
> I don't know what CRIU is and people looking at changelog entries
> shouldn't need to do a web search to find out.
>
> Could you change it a little.

Fair enough. I'll resend with more descriptive message.
But first I would like to clarify to you the problem root and why it's 
done like this.

> I'm also not sure whether to forward this (assuming the description is
> updated a little) to Al or to include it in the series to rename
> autofs4 to autofs that I'm hoping to ask be included in linux-next
> fairly soon.

Here I don't know, what's better. Of course Al can take it as well. But, 
probably, first would be nice to make sure, that this solution is the 
best one.
Description of the problem is below.

> Passing it on to Al will likely interfere with the series coming from
> linux-next so that could be bit of a hassle.
>
> Another thing I'm wondering about is the order this entry will appear
> at in the options. You order choice is sensible though and autofs
> shouldn't have a problem with the inserted option but other
> applications might.

I should put it at the end, probably?

> Finally, and perhaps most importantly, I don't get what your trying to
> do, you also haven't given any clues to that in the patch dscription.
>
> IOW how do you expect to use this.
>
> > 16.12.2015 13:02, Stanislav Kinsburskiy ÐÐÑÐÑ:
> > > This is required for CRIU to migrate a mount point, when write end
> > > in user
> > > space is closed.
> Like I said what does this mean.
>
> autofs doesn't need this when it re-constructs a mount tree from
> existing mounts on re-start or after a SIGKILL on the automount
> process.
>
> How is this different and how will it be used?
>
> The question to be answered here is "is this the best way to do it and
> will it work for the autofs mount types you expect it to"?

So, here is a brief description of the problem.
To migrate autofs mount, one have to reconstruct control pipe between 
kernel and autofs master.
There are two cases I'm wiling to support:
1) Automount binary (autofs package). This program is very gentle and it 
doesn't close write end of the pipe after mount.
2) Systemd. This program closes write end of the pipe once the mount is 
done.

The autofs restore concept is the following:
1) Mount autofs from some process with some dummy pipe.
2) Fix it's pgrp, pipe fd, timeout, etc on top of existent mount in the 
right master later (this is because of implementation of CRIU mounts 
restore, where all of them are created by one process).
What is the most important here, is that during pipe reconstruction, 
read end of it have to be placed _exactly_ on the file descriptor, which 
process has before, thus allowing to autofs master still can play it's 
role after migration.

To be able to reconstruct control pipe, one must know _exactly_ on 
_dump_ stage, which descriptor in autofs master corresponds to read end 
of the pipe, because this pipe have to be empty, because we can't (and 
don't want to) transfer some interim state in the kernel via userspace 
migration solution.
In case of systemd, this write end is already closed, so searching for 
the read end is not possible.
In case of automount write end is still there (with the same fd as in 
mount options), but one can't be sure, that this descriptor is connected 
to file structure, which is used by kernel. There can be another pipe 
end (for example, in case of systemd).

Thus, to be able to find read end of the pipe in autofs master, some 
other mark is required.
The best solution is to use pipe inode number, which allows to match 
opened pipes in a process with 100% reliability.
And the easies solution I found is to expose this number is autofs mount 
options.
If you have another better solution, I would be glad to implement it.

Hope, the above explains the problem clearly.

> > > To be able to migrate such mount, read end of the pipe have to be
> > > searched
> > > within autofs master process, and pipe inode will be used as a key.
> > >
> > > Signed-off-by: Stanislav Kinsburskiy <skinsbursky@xxxxxxxxxxxxx>
> > > ---
> > >    fs/autofs4/inode.c |    4 ++++
> > >    1 file changed, 4 insertions(+)
> > >
> > > diff --git a/fs/autofs4/inode.c b/fs/autofs4/inode.c
> > > index a3ae0b2..16f875a 100644
> > > --- a/fs/autofs4/inode.c
> > > +++ b/fs/autofs4/inode.c
> > > @@ -77,6 +77,10 @@ static int autofs4_show_options(struct seq_file
> > > *m, struct dentry *root)
> > >    		return 0;
> > >    
> > >    	seq_printf(m, ",fd=%d", sbi->pipefd);
> > > +	if (sbi->pipe)
> > > +		seq_printf(m, ",pipe_ino=%ld", sbi->pipe->f_inode
> > > ->i_ino);
> > > +	else
> > > +		seq_printf(m, ",pipe_ino=-1");
> > >    	if (!uid_eq(root_inode->i_uid, GLOBAL_ROOT_UID))
> > >    		seq_printf(m, ",uid=%u",
> > >    			from_kuid_munged(&init_user_ns,
> > > root_inode->i_uid));