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

Doug Anderson
2021-03-31T22:45:17Z

Hi,

On Wed, Mar 31, 2021 at 3:14 PM Rob Clark <robdclark@gmail.com> 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.

> @@ -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.

> + 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

-Doug