linux-kernel - Re: [PATCH v19 30/30] drm/panfrost: Switch to generic memory shrinker

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Message-ID: <20240125104932.478fa5bd@collabora.com>
Date: Thu, 25 Jan 2024 10:49:32 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc: David Airlie <airlied@...il.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Gurchetan Singh <gurchetansingh@...omium.org>, Chia-I Wu
 <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, Christian König
 <christian.koenig@....com>, Qiang Yu <yuq825@...il.com>, Steven Price
 <steven.price@....com>, Emma Anholt <emma@...olt.net>, Melissa Wen
 <mwen@...lia.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, kernel@...labora.com,
 virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v19 30/30] drm/panfrost: Switch to generic memory
 shrinker

On Fri,  5 Jan 2024 21:46:24 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -328,6 +328,7 @@ int panfrost_mmu_map(struct panfrost_gem_mapping *mapping)
>  	struct panfrost_device *pfdev = to_panfrost_device(obj->dev);
>  	struct sg_table *sgt;
>  	int prot = IOMMU_READ | IOMMU_WRITE;
> +	int ret = 0;
>  
>  	if (WARN_ON(mapping->active))
>  		return 0;
> @@ -335,15 +336,32 @@ int panfrost_mmu_map(struct panfrost_gem_mapping *mapping)
>  	if (bo->noexec)
>  		prot |= IOMMU_NOEXEC;
>  
> +	if (!obj->import_attach) {
> +		/*
> +		 * Don't allow shrinker to move pages while pages are mapped.
> +		 * It's fine to move pages afterwards because shrinker will
> +		 * take care of unmapping pages during eviction.
> +		 */

That's not exactly what this shmem_pin() is about, is it? I think it's
here to meet the drm_gem_shmem_get_pages_sgt() rule stating that pages
must be pinned while the sgt returned by drm_gem_shmem_get_pages_sgt()
is manipulated. You actually unpin the GEM just after the mmu_map_sg()
call, which means pages could very well be reclaimed while the MMU
still has a mapping referencing those physical pages. And that's fine,
because what's supposed to protect against that is the fence we
register to the GEM resv at job submission time.

> +		ret = drm_gem_shmem_pin(shmem);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	sgt = drm_gem_shmem_get_pages_sgt(shmem);
> -	if (WARN_ON(IS_ERR(sgt)))
> -		return PTR_ERR(sgt);
> +	if (WARN_ON(IS_ERR(sgt))) {
> +		ret = PTR_ERR(sgt);
> +		goto unpin;
> +	}
>  
>  	mmu_map_sg(pfdev, mapping->mmu, mapping->mmnode.start << PAGE_SHIFT,
>  		   prot, sgt);
>  	mapping->active = true;
>  
> -	return 0;
> +unpin:
> +	if (!obj->import_attach)
> +		drm_gem_shmem_unpin(shmem);
> +
> +	return ret;
>  }