linux-kernel - Re: [PATCH v20 09/10] drm/shmem-helper: Switch drm_gem_shmem

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Message-ID: <ea4f4059-7748-4bfd-9205-8e95222144da@suse.de>
Date: Wed, 2 Apr 2025 14:47:03 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>,
 Christian König <christian.koenig@....com>,
 Gerd Hoffmann <kraxel@...hat.com>, Qiang Yu <yuq825@...il.com>,
 Steven Price <steven.price@....com>,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Frank Binns <frank.binns@...tec.com>, Matt Coster <matt.coster@...tec.com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 kernel@...labora.com
Subject: Re: [PATCH v20 09/10] drm/shmem-helper: Switch
 drm_gem_shmem_vmap/vunmap to use pin/unpin

Hi

Am 22.03.25 um 22:26 schrieb Dmitry Osipenko:
> The vmapped pages shall be pinned in memory and previously get/put_pages()
> were implicitly hard-pinning/unpinning the pages. This will no longer be
> the case with addition of memory shrinker because pages_use_count > 0 won't
> determine anymore whether pages are hard-pinned (they will be soft-pinned),
> while the new pages_pin_count will do the hard-pinning. Switch the
> vmap/vunmap() to use pin/unpin() functions in a preparation of addition
> of the memory shrinker support to drm-shmem.

I've meanwhile rediscovered this patch and I'm sure this is not correct. 
Vmap should not pin AFAIK. It is possible to vmap if the buffer has been 
pinned, but that's not automatic.  For other vmaps it is necessary to 
hold the reservation lock to prevent the buffer from moving.

Best regards
Thomas

>
> Acked-by: Maxime Ripard <mripard@...nel.org>
> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> ---
>   drivers/gpu/drm/drm_gem_shmem_helper.c | 6 +++---
>   include/drm/drm_gem_shmem_helper.h     | 2 +-
>   2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 6fb96e790abd..84a196bbe44f 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -360,7 +360,7 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
>   			return 0;
>   		}
>   
> -		ret = drm_gem_shmem_get_pages_locked(shmem);
> +		ret = drm_gem_shmem_pin_locked(shmem);
>   		if (ret)
>   			goto err_zero_use;
>   
> @@ -383,7 +383,7 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
>   
>   err_put_pages:
>   	if (!drm_gem_is_imported(obj))
> -		drm_gem_shmem_put_pages_locked(shmem);
> +		drm_gem_shmem_unpin_locked(shmem);
>   err_zero_use:
>   	shmem->vmap_use_count = 0;
>   
> @@ -420,7 +420,7 @@ void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>   			return;
>   
>   		vunmap(shmem->vaddr);
> -		drm_gem_shmem_put_pages_locked(shmem);
> +		drm_gem_shmem_unpin_locked(shmem);
>   	}
>   
>   	shmem->vaddr = NULL;
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index 3a4be433d5f0..8b9bba87ae63 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -130,7 +130,7 @@ int drm_gem_shmem_madvise_locked(struct drm_gem_shmem_object *shmem, int madv);
>   static inline bool drm_gem_shmem_is_purgeable(struct drm_gem_shmem_object *shmem)
>   {
>   	return (shmem->madv > 0) &&
> -		!shmem->vmap_use_count && shmem->sgt &&
> +		!refcount_read(&shmem->pages_pin_count) && shmem->sgt &&
>   		!shmem->base.dma_buf && !drm_gem_is_imported(&shmem->base);
>   }
>   

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)