linux-kernel - Re: [PATCH v5 05/12] drm/panfrost: Check sgt to know whether pages are already mapped

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Message-ID: <20251007182045.6a239bb8@fedora>
Date: Tue, 7 Oct 2025 18:20:45 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Adrián Larumbe <adrian.larumbe@...labora.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, Steven
 Price <steven.price@....com>, kernel@...labora.com, Rob Herring
 <robh@...nel.org>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann
 <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter
 <simona@...ll.ch>
Subject: Re: [PATCH v5 05/12] drm/panfrost: Check sgt to know whether pages
 are already mapped

On Tue,  7 Oct 2025 16:01:47 +0100
Adrián Larumbe <adrian.larumbe@...labora.com> wrote:

> In the MMU's page fault ISR for a heap object, determine whether the
> faulting address belongs to a 2MiB block that was already mapped by
> checking its corresponding sgt in the Panfrost BO.
> 
> Also avoid retrieving pages from the shmem file if last one in the block
> was already present, as this means all of them had already been fetched.
> 
> This is done in preparation for a future commit in which the MMU mapping
> helper might fail, but the page array is left populated, so this cannot
> be used as a check for an early bail-out.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@...labora.com>
> ---
>  drivers/gpu/drm/panfrost/panfrost_mmu.c | 41 +++++++++++++++----------
>  1 file changed, 24 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index cf272b167feb..72864d0d478e 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -600,32 +600,39 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
>  		refcount_set(&bo->base.pages_use_count, 1);
>  	} else {
>  		pages = bo->base.pages;
> -		if (pages[page_offset]) {
> -			/* Pages are already mapped, bail out. */
> -			goto out;
> -		}
> +	}
> +
> +	sgt = &bo->sgts[page_offset / (SZ_2M / PAGE_SIZE)];
> +	if (sgt->sgl) {
> +		/* Pages are already mapped, bail out. */
> +		goto out;
>  	}
>  
>  	mapping = bo->base.base.filp->f_mapping;
>  	mapping_set_unevictable(mapping);
>  
> -	for (i = page_offset; i < page_offset + NUM_FAULT_PAGES; i++) {
> -		/* Can happen if the last fault only partially filled this
> -		 * section of the pages array before failing. In that case
> -		 * we skip already filled pages.
> +	if (!pages[page_offset + NUM_FAULT_PAGES - 1]) {
> +		/* Pages are retrieved sequentially, so if the very last
> +		 * one in the subset we want to map is already assigned, then
> +		 * there's no need to further iterate.
>  		 */

I don't think we care about optimizing the page range walk in the
unlikely case of a double fault on the same section, so I'd just keep
the existing loop unchanged.

> -		if (pages[i])
> -			continue;
> -
> -		pages[i] = shmem_read_mapping_page(mapping, i);
> -		if (IS_ERR(pages[i])) {
> -			ret = PTR_ERR(pages[i]);
> -			pages[i] = NULL;
> -			goto err_unlock;
> +		for (i = page_offset; i < page_offset + NUM_FAULT_PAGES; i++) {
> +			/* Can happen if the last fault only partially filled this
> +			 * section of the pages array before failing. In that case
> +			 * we skip already filled pages.
> +			 */
> +			if (pages[i])
> +				continue;
> +
> +			pages[i] = shmem_read_mapping_page(mapping, i);
> +			if (IS_ERR(pages[i])) {
> +				ret = PTR_ERR(pages[i]);
> +				pages[i] = NULL;
> +				goto err_unlock;
> +			}
>  		}
>  	}
>  
> -	sgt = &bo->sgts[page_offset / (SZ_2M / PAGE_SIZE)];
>  	ret = sg_alloc_table_from_pages(sgt, pages + page_offset,
>  					NUM_FAULT_PAGES, 0, SZ_2M, GFP_KERNEL);
>  	if (ret)