On Sun, 23 Jul 2023 02:47:36 +0300
Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> And new pages_pin_count field to struct drm_gem_shmem_object that will
> determine whether pages are evictable by memory shrinker. The pages will
> be evictable only when pages_pin_count=0. This patch prepares code for
> addition of the memory shrinker that will utilize the new field.
>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/drm_gem_shmem_helper.c | 9 +++++++++
> include/drm/drm_gem_shmem_helper.h | 9 +++++++++
> 2 files changed, 18 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 267153853e2c..42ba201dda50 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -274,15 +274,24 @@ static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
> dma_resv_assert_held(shmem->base.resv);
>
> ret = drm_gem_shmem_get_pages(shmem);
> + if (!ret)
> + shmem->pages_pin_count++;
>
> return ret;
> }
>
> static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
> {
> + struct drm_gem_object *obj = &shmem->base;
> +
> dma_resv_assert_held(shmem->base.resv);
>
> + if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_pin_count))
> + return;
> +
> drm_gem_shmem_put_pages(shmem);
> +
> + shmem->pages_pin_count--;
> }
>
> /**
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index bf0c31aa8fbe..7111f5743006 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -39,6 +39,15 @@ struct drm_gem_shmem_object {
> */
> unsigned int pages_use_count;
>
> + /**
> + * @pages_pin_count:
> + *
> + * Reference count on the pinned pages table.
> + * The pages allowed to be evicted by memory shrinker
> + * only when the count is zero.
> + */
> + unsigned int pages_pin_count;
Can we make it an atomic_t, so we can avoid taking the lock when the
GEM has already been pinned. That's something I need to be able to grab
a pin-ref in a path where the GEM resv lock is already held[1]. We could
of course expose the locked version, but in my case, I want to enforce
the fact the GEM has been pinned before the drm_gem_shmem_pin() call in
the section protected by the resv lock, so catching a "refcount 0 -> 1"
situation would be useful. Beside, using an atomic to avoid the
lock/unlock dance when refcount > 1 might be beneficial to everyone.
[1]https://gitlab.freedesktop.org/bbrezillon/linux/-/commit/4420fa0d5768ebdc35b34d58d4ae5fad9fbb93f9
> +
> /**
> * @madv: State for madvise
> *
