2024-03-13 12:20:18

by John Hsu

Subject: [PATCH] direct-dma: WARN_ON_ONCE when the page is not addressable by device's coherent_dma_mask

From: JohnHsu <[email protected]>

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.

Signed-off-by: JohnHsu <[email protected]>
---
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;
}

--
2.18.0

2024-03-13 12:52:00

by Robin Murphy

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 <[email protected]>
>
> 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 <[email protected]>
> ---
> 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;
> }
>