linux-kernel - Re: [PATCH v2 2/4] drm/msm: Avoid mutex in shrinker

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Date:   Thu, 1 Apr 2021 08:34:43 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>
Subject: Re: [PATCH v2 2/4] drm/msm: Avoid mutex in shrinker_count()

Hi,

On Wed, Mar 31, 2021 at 6:24 PM Rob Clark <robdclark@...il.com> wrote:
>
> @@ -45,6 +30,9 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
>         list_for_each_entry(msm_obj, &priv->inactive_dontneed, mm_list) {
>                 if (freed >= sc->nr_to_scan)
>                         break;
> +               /* Use trylock, because we cannot block on a obj that
> +                * might be trying to acquire mm_lock
> +                */

nit: I thought the above multi-line commenting style was only for
"net" subsystem?

>                 if (!msm_gem_trylock(&msm_obj->base))
>                         continue;
>                 if (is_purgeable(msm_obj)) {
> @@ -56,8 +44,11 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
>
>         mutex_unlock(&priv->mm_lock);
>
> -       if (freed > 0)
> +       if (freed > 0) {
>                 trace_msm_gem_purge(freed << PAGE_SHIFT);
> +       } else {
> +               return SHRINK_STOP;
> +       }

It probably doesn't matter, but I wonder if we should still be
returning SHRINK_STOP if we got any trylock failures. It could
possibly be worth returning 0 in that case?


> @@ -75,6 +66,9 @@ vmap_shrink(struct list_head *mm_list)
>         unsigned unmapped = 0;
>
>         list_for_each_entry(msm_obj, mm_list, mm_list) {
> +               /* Use trylock, because we cannot block on a obj that
> +                * might be trying to acquire mm_lock
> +                */

If you end up changing the commenting style above, should also be here.

At this point this seems fine to land to me. Though I'm not an expert
on every interaction in this code, I've spent enough time starting at
it that I'm comfortable with:

Reviewed-by: Douglas Anderson <dianders@...omium.org>