Re: [PATCH] drm/xe/dma_buf: stop relying on placement in unmap

[Matthew Auld <matthew.auld@intel.com>]

On 03/04/2025 15:07, Matthew Auld wrote:
> The is_vram() is checking the current placement, however if we consider
> exported VRAM with dynamic dma-buf, it looks possible for the xe driver
> to async evict the memory, notifying the importer, however importer does
> not have to call unmap_attachment() immediately, but rather just as
> "soon as possible", like when the dma-resv idles. Following from this we
> would then pipeline the move, attaching the fence to the manager, and
> then update the current placement. But when the unmap_attachment() runs
> at some later point we might see that is_vram() is now false, and take
> the complete wrong path when dma-unmapping the sg, leading to
> explosions.
> 
> To fix this rather make a note in the attachment if the sg was
> originally mapping vram or tt pages.
> 
> Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/4563
> Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: <stable@vger.kernel.org> # v6.8+
> ---
>   drivers/gpu/drm/xe/xe_dma_buf.c | 23 ++++++++++++++++++++---
>   1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c b/drivers/gpu/drm/xe/xe_dma_buf.c
> index f67803e15a0e..b71058e26820 100644
> --- a/drivers/gpu/drm/xe/xe_dma_buf.c
> +++ b/drivers/gpu/drm/xe/xe_dma_buf.c
> @@ -22,6 +22,22 @@
>   
>   MODULE_IMPORT_NS("DMA_BUF");
>   
> +/**
> + * struct xe_sg_info - Track the exported sg info
> + */
> +struct xe_sg_info {
> +	/** @is_vram: True if this sg is mapping VRAM. */
> +	bool is_vram;
> +};
> +
> +static struct xe_sg_info tt_sg_info = {
> +	.is_vram = false,
> +};
> +
> +static struct xe_sg_info vram_sg_info = {
> +	.is_vram = true,
> +};
> +
>   static int xe_dma_buf_attach(struct dma_buf *dmabuf,
>   			     struct dma_buf_attachment *attach)
>   {
> @@ -118,6 +134,7 @@ static struct sg_table *xe_dma_buf_map(struct dma_buf_attachment *attach,
>   		if (dma_map_sgtable(attach->dev, sgt, dir,
>   				    DMA_ATTR_SKIP_CPU_SYNC))
>   			goto error_free;
> +		attach->priv = &tt_sg_info;
>   		break;
>   
>   	case XE_PL_VRAM0:
> @@ -128,6 +145,7 @@ static struct sg_table *xe_dma_buf_map(struct dma_buf_attachment *attach,
>   					      dir, &sgt);
>   		if (r)
>   			return ERR_PTR(r);
> +		attach->priv = &vram_sg_info;

Maybe we need to subclass the sg itself? It looks possible to call map 
again, before the unmap, and you might get different memory if you had 
mixed placement bo...

>   		break;
>   	default:
>   		return ERR_PTR(-EINVAL);
> @@ -145,10 +163,9 @@ static void xe_dma_buf_unmap(struct dma_buf_attachment *attach,
>   			     struct sg_table *sgt,
>   			     enum dma_data_direction dir)
>   {
> -	struct dma_buf *dma_buf = attach->dmabuf;
> -	struct xe_bo *bo = gem_to_xe_bo(dma_buf->priv);
> +	struct xe_sg_info *sg_info = attach->priv;
>   
> -	if (!xe_bo_is_vram(bo)) {
> +	if (!sg_info->is_vram) {
>   		dma_unmap_sgtable(attach->dev, sgt, dir, 0);
>   		sg_free_table(sgt);
>   		kfree(sgt);