Re: [PATCH 03/11] fs: add new read_uptr and write_uptr file operations

[Christoph Hellwig <hch@lst.de>]

On Wed, Jun 24, 2020 at 07:29:44PM +0100, Matthew Wilcox wrote:
> On Wed, Jun 24, 2020 at 08:24:37PM +0200, Christoph Hellwig wrote:
> > On Wed, Jun 24, 2020 at 11:20:26AM -0700, Linus Torvalds wrote:
> > > On Wed, Jun 24, 2020 at 11:14 AM Christoph Hellwig <hch@lst.de> wrote:
> > > >
> > > > So we'd need new user copy functions for just those cases
> > > 
> > > No. We'd open-code them. They'd look at "oh, I'm supposed to use a
> > > kernel pointer" and just use those.
> > > 
> > > IOW, basically IN THE CODE that cares (and the whole argument is that
> > > this code is one or two special cases) you do
> > > 
> > >     /* This has not been converted to the new world order */
> > >     if (get_fs() == KERNEL_DS) memcpy(..) else copy_from_user();
> > > 
> > > You're overdesigning things. You're making them more complex than they
> > > need to be.
> > 
> > I wish it was so simple.  I really don't like overdesigns, trust me.
> > 
> > But please take a look at setsockopt and all the different instances
> > (count 90 .setsockopt wireups, and they then branch out into
> > various subroutines as well).  I really don't want to open code that
> > there, but we could do helper specific to setsockopt.
> 
> Can we do a setsockopt_iter() which replaces optval/optlen with an iov_iter?

We could.  The only downside is int-sized sockopts are common, and used
in the fast path of networking applications (e.g. cork,uncork) and this
might introduce enough overhead to be noticable.