Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

Al Viro
2020-06-13T15:41:28Z

On Sat, Jun 13, 2020 at 04:31:02PM +0100, Al Viro wrote:
> On Sat, Jun 13, 2020 at 07:12:36PM +0530, afzal mohammed wrote:
> > Hi,
> >
> > On Sat, Jun 13, 2020 at 01:56:15PM +0100, Al Viro wrote:
> >
> > > Incidentally, what about get_user()/put_user()? _That_ is where it's
> > > going to really hurt...
> >
> > All other uaccess routines are also planned to be added, posting only
> > copy_{from,to}_user() was to get early feedback (mentioned in the
> > cover letter)
>
> Sure, but what I mean is that I'd expect the performance loss to be
> dominated by that, not by copy_from_user/copy_to_user on large amounts
> of data. Especially on the loads like kernel builds - a lot of stat()
> and getdents() calls there.

To clarify: stat() means small copy_to_user(), getdents() - a mix of
put_user() and small copy_to_user(). I would be very surprised if it
does not hurt a lot.