Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

From: Daniel Vetter
Date: Sun Sep 20 2020 - 04:24:21 EST

On Sun, Sep 20, 2020 at 08:23:26AM +0200, Thomas Gleixner wrote:
> On Sat, Sep 19 2020 at 12:37, Daniel Vetter wrote:
> > On Sat, Sep 19, 2020 at 12:35 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
> >> I think it should be the case, but I want to double check: Will
> >> copy_*_user be allowed within a kmap_temporary section? This would
> >> allow us to ditch an absolute pile of slowpaths.
> >
> > (coffee just kicked in) copy_*_user is ofc allowed, but if you hit a
> > page fault you get a short read/write. This looks like it would remove
> > the need to handle these in a slowpath, since page faults can now be
> > served in this new kmap_temporary sections. But this sounds too good
> > to be true, so I'm wondering what I'm missing.
> In principle we could allow pagefaults, but not with the currently
> proposed interface which can be called from any context. Obviously if
> called from atomic context it can't handle user page faults.

Yeah that's clear, but does the implemention need to disable pagefaults

> In theory we could make a variant which does not disable pagefaults, but
> that's what kmap() already provides.

Currently we have a bunch of code which roughly does


if (short_copy_user) {

And the only reason is that kmap is a notch slower, hence the fastpath. If
we get a kmap which is fast and allows pagefaults (only in contexts that
allow pagefaults already ofc) then we can ditch a pile of kmap users.

Cheers, Daniel
Daniel Vetter
Software Engineer, Intel Corporation