linux-kernel - Re: [PATCH 03/10] swiotlb: do not panic on mapping failures

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Date:   Thu, 18 Oct 2018 20:17:14 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/10] swiotlb: do not panic on mapping failures

On Mon, Oct 08, 2018 at 10:02:39AM +0200, Christoph Hellwig wrote:
> All properly written drivers now have error handling in the
> dma_map_single / dma_map_page callers.  As swiotlb_tbl_map_single already
> prints a useful warning when running out of swiotlb pool swace we can

s/swace/space/

> also remove swiotlb_full entirely as it serves no purpose now.

The panic albeit was useful for those drivers that didn't handle it.

Thoughts? I am kind of thinking screw them but then I am not too thrilled
about silent data corruption errors.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  kernel/dma/swiotlb.c | 33 +--------------------------------
>  1 file changed, 1 insertion(+), 32 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 26d3af52956f..69bf305ee5f8 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -761,34 +761,6 @@ static bool swiotlb_free_buffer(struct device *dev, size_t size,
>  	return true;
>  }
>  
> -static void
> -swiotlb_full(struct device *dev, size_t size, enum dma_data_direction dir,
> -	     int do_panic)
> -{
> -	if (swiotlb_force == SWIOTLB_NO_FORCE)
> -		return;
> -
> -	/*
> -	 * Ran out of IOMMU space for this operation. This is very bad.
> -	 * Unfortunately the drivers cannot handle this operation properly.
> -	 * unless they check for dma_mapping_error (most don't)
> -	 * When the mapping is small enough return a static buffer to limit
> -	 * the damage, or panic when the transfer is too big.
> -	 */
> -	dev_err_ratelimited(dev, "DMA: Out of SW-IOMMU space for %zu bytes\n",
> -			    size);
> -
> -	if (size <= io_tlb_overflow || !do_panic)
> -		return;
> -
> -	if (dir == DMA_BIDIRECTIONAL)
> -		panic("DMA: Random memory could be DMA accessed\n");
> -	if (dir == DMA_FROM_DEVICE)
> -		panic("DMA: Random memory could be DMA written\n");
> -	if (dir == DMA_TO_DEVICE)
> -		panic("DMA: Random memory could be DMA read\n");
> -}
> -
>  /*
>   * Map a single buffer of the indicated size for DMA in streaming mode.  The
>   * physical address to use is returned.
> @@ -817,10 +789,8 @@ dma_addr_t swiotlb_map_page(struct device *dev, struct page *page,
>  
>  	/* Oh well, have to allocate and map a bounce buffer. */
>  	map = map_single(dev, phys, size, dir, attrs);
> -	if (map == SWIOTLB_MAP_ERROR) {
> -		swiotlb_full(dev, size, dir, 1);
> +	if (map == SWIOTLB_MAP_ERROR)
>  		return __phys_to_dma(dev, io_tlb_overflow_buffer);
> -	}
>  
>  	dev_addr = __phys_to_dma(dev, map);
>  
> @@ -948,7 +918,6 @@ swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl, int nelems,
>  			if (map == SWIOTLB_MAP_ERROR) {
>  				/* Don't panic here, we expect map_sg users
>  				   to do proper error handling. */
> -				swiotlb_full(hwdev, sg->length, dir, 0);
>  				attrs |= DMA_ATTR_SKIP_CPU_SYNC;
>  				swiotlb_unmap_sg_attrs(hwdev, sgl, i, dir,
>  						       attrs);
> -- 
> 2.19.0
>