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

[Robin Murphy]

On 26/09/17 14:24, miles.chen@xxxxxxxxxxxx wrote:
> From: Miles Chen <miles.chen@xxxxxxxxxxxx>
> 
> dma-debug report 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()
> always use type "dma_debug_coherent" and assume that dma_alloc_coherent()
> always returns a linear address.
> 
> However if a device returns false on is_device_dma_coherent(),
> dma_alloc_coherent() will create another non-cacheable mapping
> (also non linear). 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().
> 
> Fix this by calculating correct pfn if is_device_dma_coherent()
> returns false.

As the inevitable storm of kbuild robot reports will tell you soon, you
can't do that: is_device_dma_coherent() is a private helper between
arm{,64} arch code and xen, and should not be used anywhere else.

> Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> ---
>  lib/dma-debug.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> index ea4cc3d..b17e56e 100644
> --- a/lib/dma-debug.c
> +++ b/lib/dma-debug.c
> @@ -47,6 +47,8 @@ enum {
>  	dma_debug_sg,
>  	dma_debug_coherent,
>  	dma_debug_resource,
> +	dma_debug_noncoherent,
> +	nr_dma_debug_types,
>  };
>  
>  enum map_err_types {
> @@ -154,9 +156,9 @@ static inline bool dma_debug_disabled(void)
>  	[MAP_ERR_CHECKED] = "dma map error checked",
>  };
>  
> -static const char *type2name[5] = { "single", "page",
> +static const char *type2name[nr_dma_debug_types] = { "single", "page",
>  				    "scather-gather", "coherent",
> -				    "resource" };
> +				    "resource", "noncoherent" };
>  
>  static const char *dir2name[4] = { "DMA_BIDIRECTIONAL", "DMA_TO_DEVICE",
>  				   "DMA_FROM_DEVICE", "DMA_NONE" };
> @@ -1484,6 +1486,7 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
>  			      dma_addr_t dma_addr, void *virt)
>  {
>  	struct dma_debug_entry *entry;
> +	bool coherent = is_device_dma_coherent(dev);
>  
>  	if (unlikely(dma_debug_disabled()))
>  		return;
> @@ -1495,9 +1498,11 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
>  	if (!entry)
>  		return;
>  
> -	entry->type      = dma_debug_coherent;
> +	entry->type      = coherent ? dma_debug_coherent :
> +					dma_debug_noncoherent;
>  	entry->dev       = dev;
> -	entry->pfn	 = page_to_pfn(virt_to_page(virt));

There are more architectures where the virtual address returned by
dma_alloc_coherent is not a linear map address - some just have a static
offset between cacheable and non-cacheable aliases - so there may be
other cases where this is the wrong calculation, but at least those
probably don't trigger the problematic false-positive.

That said, the cases where coherent allocations *are* dynamically
remapped should be easy enough to handle properly without having to
resort to dodgy hacks:

	if (is_vmalloc_addr(virt))
		pfn = vmalloc_to_pfn(virt);
	else
		pfn = page_to_pfn(virt_to_page(virt));

Simple.

> +	entry->pfn	 = coherent ? page_to_pfn(virt_to_page(virt)) :
> +					dma_addr >> PAGE_SHIFT;

And in particular, this is just as likely to just give *different* false
positives, since there's no guarantee whatsoever that dma_addr has any
relationship to the appropriate pfn.

Robin.

>  	entry->offset	 = offset_in_page(virt);
>  	entry->size      = size;
>  	entry->dev_addr  = dma_addr;
> @@ -1510,10 +1515,13 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
>  void debug_dma_free_coherent(struct device *dev, size_t size,
>  			 void *virt, dma_addr_t addr)
>  {
> +	bool coherent = is_device_dma_coherent(dev);
>  	struct dma_debug_entry ref = {
> -		.type           = dma_debug_coherent,
> +		.type           = coherent ? dma_debug_coherent :
> +						dma_debug_noncoherent,
>  		.dev            = dev,
> -		.pfn		= page_to_pfn(virt_to_page(virt)),
> +		.pfn		= coherent ? page_to_pfn(virt_to_page(virt)) :
> +						addr >> PAGE_SHIFT,
>  		.offset		= offset_in_page(virt),
>  		.dev_addr       = addr,
>  		.size           = size,
>