Re: [PATCH v3] dma-debug: fix incorrect pfn calculation

[Robin Murphy]

[+DMA maintainers]

On 27/09/17 04:48, miles.chen@xxxxxxxxxxxx wrote:
> From: Miles Chen <miles.chen@xxxxxxxxxxxx>
> 
> dma-debug reports the following warning:
> 
> [name:panic&]WARNING: CPU: 3 PID: 298 at kernel-4.4/lib/dma-debug.c:604
> debug _dma_assert_idle+0x1a8/0x230()
> DMA-API: cpu touching an active dma mapped cacheline [cln=0x00000882300]
> CPU: 3 PID: 298 Comm: vold Tainted: G        W  O    4.4.22+ #1
> Hardware name: MT6739 (DT)
> Call trace:
> [<ffffff800808acd0>] dump_backtrace+0x0/0x1d4
> [<ffffff800808affc>] show_stack+0x14/0x1c
> [<ffffff800838019c>] dump_stack+0xa8/0xe0
> [<ffffff80080a0594>] warn_slowpath_common+0xf4/0x11c
> [<ffffff80080a061c>] warn_slowpath_fmt+0x60/0x80
> [<ffffff80083afe24>] debug_dma_assert_idle+0x1a8/0x230
> [<ffffff80081dca9c>] wp_page_copy.isra.96+0x118/0x520
> [<ffffff80081de114>] do_wp_page+0x4fc/0x534
> [<ffffff80081e0a14>] handle_mm_fault+0xd4c/0x1310
> [<ffffff8008098798>] do_page_fault+0x1c8/0x394
> [<ffffff800808231c>] do_mem_abort+0x50/0xec
> 
> I found that debug_dma_alloc_coherent() and debug_dma_free_coherent()
> assume that dma_alloc_coherent() always returns a linear address.
> However it's possible that dma_alloc_coherent() returns a non-linear
> address. In this case, page_to_pfn(virt_to_page(virt)) will return an
> incorrect pfn. If the pfn is valid and mapped as a COW page,
> we will hit the warning when doing wp_page_copy().

Yeah, we definitely want that explanation recorded in the commit, since
the warning is pretty non-obvious otherwise.

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

> Fix this by calculating pfn for linear and non-linear addresses.
> 
> Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> ---
>  lib/dma-debug.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> index ea4cc3d..e5b4237 100644
> --- a/lib/dma-debug.c
> +++ b/lib/dma-debug.c
> @@ -1497,7 +1497,8 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
>  
>  	entry->type      = dma_debug_coherent;
>  	entry->dev       = dev;
> -	entry->pfn	 = page_to_pfn(virt_to_page(virt));
> +	entry->pfn	 = is_vmalloc_addr(virt) ? vmalloc_to_pfn(virt) :
> +						page_to_pfn(virt_to_page(virt));
>  	entry->offset	 = offset_in_page(virt);
>  	entry->size      = size;
>  	entry->dev_addr  = dma_addr;
> @@ -1513,7 +1514,8 @@ void debug_dma_free_coherent(struct device *dev, size_t size,
>  	struct dma_debug_entry ref = {
>  		.type           = dma_debug_coherent,
>  		.dev            = dev,
> -		.pfn		= page_to_pfn(virt_to_page(virt)),
> +		.pfn		= is_vmalloc_addr(virt) ? vmalloc_to_pfn(virt) :
> +						page_to_pfn(virt_to_page(virt)),
>  		.offset		= offset_in_page(virt),
>  		.dev_addr       = addr,
>  		.size           = size,
>