Re: [PATCH 1/5] drm/i915: Round up object allocations

[Daniel Vetter <daniel@ffwll.ch>]

On Sun, Jan 26, 2014 at 11:00:52AM -0800, Ben Widawsky wrote:
> On Sun, Jan 26, 2014 at 10:07:45AM +0000, Chris Wilson wrote:
> > On Sun, Jan 26, 2014 at 11:01:50AM +0100, Daniel Vetter wrote:
> > > On Sun, Jan 26, 2014 at 09:57:08AM +0000, Chris Wilson wrote:
> > > > On Sun, Jan 26, 2014 at 10:17:57AM +0100, Daniel Vetter wrote:
> > > > > On Sun, Jan 26, 2014 at 6:49 AM, Ben Widawsky <ben@bwidawsk.net> wrote:
> > > > > > On Sat, Jan 25, 2014 at 09:28:24PM +0100, Daniel Vetter wrote:
> > > > > >> On Thu, Jan 23, 2014 at 07:21:10PM -0800, Ben Widawsky wrote:
> > > > > >> > DRM gets very mad when you request an object which occupies a partial
> > > > > >> > page. As a DRM driver, i915 never really wants to anger DRM, and would
> > > > > >> > always just want the rounding done for us.
> > > > > >> >
> > > > > >> > Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> > > > > >> > ---
> > > > > >> >  drivers/gpu/drm/i915/i915_gem.c | 2 ++
> > > > > >> >  1 file changed, 2 insertions(+)
> > > > > >> >
> > > > > >> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > > > > >> > index 024e454..8cd1134 100644
> > > > > >> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > > > > >> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > > > > >> > @@ -4168,6 +4168,8 @@ struct drm_i915_gem_object *i915_gem_alloc_object(struct drm_device *dev,
> > > > > >> >     struct address_space *mapping;
> > > > > >> >     gfp_t mask;
> > > > > >> >
> > > > > >> > +   size = round_up(size, PAGE_SIZE);
> > > > > >> > +
> > > > > >>
> > > > > >> Nope, if there's some code that doesn't do page-aligend bo allocations it
> > > > > >> needs to be fixed there. If you want throw a WARN_ON and early return in
> > > > > >> here.
> > > > > >> -Daniel
> > > > > >
> > > > > > Why?
> > > > > 
> > > > > Because allocating a non-page aligned gem bo is a bug. All current
> > > > > in-kernel allocations are already aligned. I've thought that we also
> > > > > reject unaligned request from userspace but apparently we help out
> > > > > since forever (i.e. gem was merged). Might be worth a shot to turn
> > > > > that into an -EINVAL if libdrm does the right rounding ...
> > > > 
> > > > We already have an -EINVAL guard on our create ioctl(s).
> > > 
> > > Only for size == 0, not for non-aligned size, at least afaics (no coffee
> > > yet ...).
> > 
> > The lack of caffeine was mine. I say if (blah(PAGE_SIZE)) return -EINVAL
> > and was happy.
> > 
> > But as it happens it still implies that the BUG_ON in drm/drm_gem.c is
> > relevant as is and we shouldn't be papering over it.
> > -Chris
> > 
> 
> I was referring to internal, driver callers. If all internal callers
> round up, we may as well do it for them. If the earlier assertion is
> that we block bad sizes already at the IOCTL hanlder... then what's the
> problem again?

create2_ioctl should imo reject non-page-aligned requests. I guess we
could dig through libdrm history and check whether that's also possible
for the current create ioctl.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch