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

[Al Viro]

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.