linux-kernel - Re: [PATCH] direct-dma: WARN_ON_ONCE when the page is not addressable by device's coherent_dma

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Date: Wed, 13 Mar 2024 12:43:52 +0000
From: Robin Murphy <robin.murphy@....com>
To: John Hsu <john.hsu@...iatek.com>, Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 Matthias Brugger <matthias.bgg@...il.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
Cc: surenb@...gle.com, Chinwen Chang <chinwen.change@...iatek.com>,
 Casper Li <casper.li@...iatek.com>,
 Kuan-Ying Lee <Kuan-Ying.Lee@...iatek.com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] direct-dma: WARN_ON_ONCE when the page is not addressable
 by device's coherent_dma_mask

On 2024-03-13 12:19 pm, John Hsu wrote:
> From: JohnHsu <john.hsu@...iatek.com>
> 
> The dma_direct_alloc() may return null in some cases. For example, the
> allocated page is not addressable for the device's coherent_dma_mask,
> and the allocated page will be assigned to null.
> 
> This patch can WARN_ON_ONCE() when the returned page is null in
> dma_direct_alloc. It helps the developers position the root cause of
> allocation failure rapidly.

No. Failure to allocate a buffer can happen for any number of reasons, 
it is not specific to dma-direct, and in some cases it is even expected, 
hence why DMA_ATTR_NO_WARN exists. And either way it's still not a 
condition worthy of panicking when panic_on_warn is in use.

Sure, this may well be a handy development hack for debugging a 
particular driver which isn't handling failure correctly, but it is not 
suitable for mainline.

Thanks,
Robin.

> Signed-off-by: JohnHsu <john.hsu@...iatek.com>
> ---
>   kernel/dma/direct.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 9596ae1aa0da..a73b8ad1ef9e 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -156,6 +156,8 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
>   		}
>   	}
>   
> +	WARN_ON_ONCE(!page);
> +
>   	return page;
>   }
>