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

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Date:   Thu, 18 Oct 2018 20:18:03 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....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 Thu, Oct 11, 2018 at 07:06:31PM +0100, Robin Murphy wrote:
> On 08/10/18 09:02, 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
> > also remove swiotlb_full entirely as it serves no purpose now.
> 
> $ git grep -l 'dma_map_\(page\|single\)' drivers/ | wc -l
> 385
> $ git grep -l dma_mapping_error drivers/ | wc -l
> 396
> 
> Close enough, to first approximation :D
> 
> I agree the other (non-fatal) warning seems sufficient, and frankly the
> panic can be a bit of a pain for development sometimes.

What would you recommend instead?
> 
> Reviewed-by: Robin Murphy <robin.murphy@....com>
> 
> > 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);
> >