Re: [PATCH] drm/i915: Retry gtt fault when out of fence register

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Oct 12, 2023 at 06:12:26PM +0200, Greg KH wrote:
> On Thu, Oct 12, 2023 at 04:53:38PM +0300, Ville Syrjälä wrote:
> > On Thu, Oct 12, 2023 at 03:40:08PM +0200, Greg KH wrote:
> > > On Thu, Oct 12, 2023 at 04:28:01PM +0300, Ville Syrjala wrote:
> > > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > 
> > > > If we can't find a free fence register to handle a fault in the GMADR
> > > > range just return VM_FAULT_NOPAGE without populating the PTE so that
> > > > userspace will retry the access and trigger another fault. Eventually
> > > > we should find a free fence and the fault will get properly handled.
> > > > 
> > > > A further improvement idea might be to reserve a fence (or one per CPU?)
> > > > for the express purpose of handling faults without having to retry. But
> > > > that would require some additional work.
> > > > 
> > > > Looks like this may have gotten broken originally by
> > > > commit 39965b376601 ("drm/i915: don't trash the gtt when running out of fences")
> > > > as that changed the errno to -EDEADLK which wasn't handle by the gtt
> > > > fault code either. But later in commit 2feeb52859fc ("drm/i915/gt: Fix
> > > > -EDEADLK handling regression") I changed it again to -ENOBUFS as -EDEADLK
> > > > was now getting used for the ww mutex dance. So this fix only makes
> > > > sense after that last commit.
> > > > 
> > > > Cc: stable@vger.kernel.org
> > > > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9479
> > > > Fixes: 2feeb52859fc ("drm/i915/gt: Fix -EDEADLK handling regression")
> > > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/gem/i915_gem_mman.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > 
> > > <formletter>
> > > 
> > > This is not the correct way to submit patches for inclusion in the
> > > stable kernel tree.  Please read:
> > >     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> > > for how to do this properly.
> > > 
> > > </formletter>
> > 
> > Say what now?
> 
> Sorry, my bot thought this was a patch sent only to stable, I've kicked
> it a bit and it shouldn't do that again...

Ah OK, thanks.

I was a bit worried that my reading comprehension had deterirated enough
that I couldn't figure iut what new requirement in the process I had
violated :)

-- 
Ville Syrjälä
Intel