[Intel-gfx] [PATCH 1/3] drm/i915: Use pagecache write to prepopulate shmemfs from pwrite-ioctl

[Chris Wilson]

On Mon, Mar 06, 2017 at 01:49:33PM +0000, Tvrtko Ursulin wrote:
> 
> On 06/03/2017 12:48, Chris Wilson wrote:
> >For the moment, yes :) This was a quick hack to hide a regression - the
> 
> Oh it was a real regression and not just an optimisation for a
> strange client behaviour? You should add a Fixes: tag then. And
> explain in the commit what it was (the regression).

I was having that debate with myself in another thread. userspace is
clearly broken, but this excerbates the damage.
 
> >real bug fix will be breaking struct_mutex out of the shrinker, I think,
> >and there's some nasty behaviour to resolve when we do hit the shrinker
> >the whole object page-in/page-out behaviour is much, much slower than
> >what should be the equivalent with individual pages via shmemfs. The
> >bonus here is that shmemfs can avoid clearing/swapping-in the page if
> >knows it will be completely overwritten, which makes the patch useful on
> >its own merit.
> 
> So in this particular case is that becuase it is swapping out even
> the untouched pages? And it started doing that recently after some
> commit or what?

Remember when I said that nobody would touch pages without using them,
(and so could defer the update for the shrinker until we had the
struct_mutex) and certainly not 16GiB of written-but-unused pages on a
small box? libva happened.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre