Re: [PATCH 2/4] drm/msm: Avoid mutex in shrinker_count()

[Rob Clark]

On Wed, Mar 31, 2021 at 3:44 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Wed, Mar 31, 2021 at 3:14 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
> >
> > @@ -818,11 +820,19 @@ static void update_inactive(struct msm_gem_object *msm_obj)
> >         mutex_lock(&priv->mm_lock);
> >         WARN_ON(msm_obj->active_count != 0);
> >
> > +       if (msm_obj->dontneed)
> > +               mark_unpurgable(msm_obj);
> > +
> >         list_del_init(&msm_obj->mm_list);
> > -       if (msm_obj->madv == MSM_MADV_WILLNEED)
> > +       if (msm_obj->madv == MSM_MADV_WILLNEED) {
> >                 list_add_tail(&msm_obj->mm_list, &priv->inactive_willneed);
> > -       else
> > +       } else if (msm_obj->madv == MSM_MADV_DONTNEED) {
> >                 list_add_tail(&msm_obj->mm_list, &priv->inactive_dontneed);
> > +               mark_purgable(msm_obj);
> > +       } else {
> > +               WARN_ON(msm_obj->madv != __MSM_MADV_PURGED);
> > +               list_add_tail(&msm_obj->mm_list, &priv->inactive_purged);
>
> I'm probably being dense, but what's the point of adding it to the
> "inactive_purged" list here? You never look at that list, right? You
> already did a list_del_init() on this object's list pointer
> ("mm_list"). I don't see how adding it to a bogus list helps with
> anything.

It preserves the "every bo is in one of these lists" statement, but
other than that you are right we aren't otherwise doing anything with
that list.  (Or we could replace the list_del_init() with list_del()..
I tend to instinctively go for list_del_init())

>
> > @@ -198,6 +203,33 @@ static inline bool is_vunmapable(struct msm_gem_object *msm_obj)
> >         return (msm_obj->vmap_count == 0) && msm_obj->vaddr;
> >  }
> >
> > +static inline void mark_purgable(struct msm_gem_object *msm_obj)
> > +{
> > +       struct msm_drm_private *priv = msm_obj->base.dev->dev_private;
> > +
> > +       WARN_ON(!mutex_is_locked(&priv->mm_lock));
> > +
> > +       if (WARN_ON(msm_obj->dontneed))
> > +               return;
>
> The is_purgeable() function also checks other things besides just
> "MSM_MADV_DONTNEED". Do we need to check those too? Specifically:
>
>  msm_obj->sgt && !msm_obj->base.dma_buf && !msm_obj->base.import_attach
>
> ...or is it just being paranoid?
>
> I guess I'm just worried that if any of those might be important then
> we'll consistently report back that we have a count of things that can
> be purged but then scan() won't find anything to do. That wouldn't be
> great.

Hmm, I thought msm_gem_madvise() returned an error instead of allowing
MSM_MADV_DONTNEED to be set on imported/exported dma-bufs.. it
probably should to be complete (but userspace already knows not to
madvise an imported/exported buffer for other reasons.. ie. we can't
let a shared buffer end up in the bo cache).  I'll re-work that a bit.

The msm_obj->sgt case is a bit more tricky.. that will be the case of
a freshly allocated obj that does not have backing patches yet.  But
it seems like enough of a corner case, that I'm happy to live with
it.. ie. the tricky thing is not leaking decrements of
priv->shrinkable_count or underflowing priv->shrinkable_count, and
caring about the !msm_obj->sgt case doubles the number of states an
object can be in, and the shrinker->count() return value is just an
estimate.

>
> > +       priv->shrinkable_count += msm_obj->base.size >> PAGE_SHIFT;
> > +       msm_obj->dontneed = true;
> > +}
> > +
> > +static inline void mark_unpurgable(struct msm_gem_object *msm_obj)
> > +{
> > +       struct msm_drm_private *priv = msm_obj->base.dev->dev_private;
> > +
> > +       WARN_ON(!mutex_is_locked(&priv->mm_lock));
> > +
> > +       if (WARN_ON(!msm_obj->dontneed))
> > +               return;
> > +
> > +       priv->shrinkable_count -= msm_obj->base.size >> PAGE_SHIFT;
> > +       WARN_ON(priv->shrinkable_count < 0);
>
> If you changed the order maybe you could make shrinkable_count
> "unsigned long" to match the shrinker API?
>
>  new_shrinkable = msm_obj->base.size >> PAGE_SHIFT;
>  WARN_ON(new_shrinkable > priv->shrinkable_count);
>  priv->shrinkable_count -= new_shrinkable
>

True, although I've developed a preference for signed integers in
cases where it can underflow if you mess up

BR,
-R