Re: [PATCH 0/1] ns: introduce proc_get_ns_by_fd()

[Konstantin Khlebnikov]

On 25.09.2015 20:56, Oleg Nesterov wrote:
> On 09/25, Konstantin Khlebnikov wrote:
> > +struct ns_common *proc_ns_fdget(int fd, int nstype, struct fd *fd_ref)
> >   {
> > -	struct file *file;
> > +	struct ns_common *ns;
> > +	struct fd f;
> >
> > -	file = fget(fd);
> > -	if (!file)
> > +	f = fdget(fd);
> > +	if (!f.file)
> >   		return ERR_PTR(-EBADF);
> >
> > -	if (file->f_op != &ns_file_operations)
> > +	if (f.file->f_op != &ns_file_operations)
> > +		goto out_invalid;
> > +
> > +	ns = get_proc_ns(file_inode(f.file));
> > +	if (nstype && (ns->ops->type != nstype))
> >   		goto out_invalid;
> >
> > -	return file;
> > +	*fd_ref = f;
> > +	return ns;
> >
> >   out_invalid:
> > -	fput(file);
> > +	fdput(f);
> >   	return ERR_PTR(-EINVAL);
> >   }
>
> Well yes, fdget() makes sense but this is minor.
>
> Honestly, I do not really like the new helper... I understand this
> is subjective, so I won't insist. But how about 1/1? We do not need
> fd/file at all. With this patch your sys_getvpid() can just use
> proc_get_ns_by_fd(fd, CLONE_NEWPID) and put_pid_ns().

Hmm. My version has 0 or 2 atomic ops per get-put sequence.
Your version: 2 or 4 atomic ops. Plus even in worst case pinning by
struct fd theoretically scales better because it touches refcount at
struct file: there're might be many of them for one namespace.

> Eric, what do you think?
>
> See also "TODO" in the changelog.
>
> Oleg.


--
Konstantin
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