Re: [PATCH] kernel: dma: let dma use vmalloc area

[Robin Murphy <robin.murphy@arm.com>]

On 2023-11-27 8:56 am, Zhaoyang Huang wrote:
> On Mon, Nov 27, 2023 at 3:14 PM Christoph Hellwig <hch@lst.de> wrote:
>>
>> On Mon, Nov 27, 2023 at 11:09:30AM +0800, zhaoyang.huang wrote:
>>> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
>>>
>>> memremap within dma_init_coherent_memory will map the given phys_addr
>>> into vmalloc area if the pa is not found during iterating iomem_resources,
>>> which conflict the rejection of vmalloc area in dma_map_single_attrs.
>>
>> I can't parse this sentence.
> Sorry for the confusion, please find below codes for more information.
> dma_init_coherent_memory
>      memremap
>          addr = ioremap_wt(offset, size);
> What I mean is addr is a vmalloc address, which is implicitly mapped
> by dma's framework and not be aware of to the driver.
>>
>>> IMO, it is find to let all valid virtual address be valid for DMA as the
>>> user will keep corresponding RAM safe for transfer.
>>
>> No, vmalloc address can't be passed to map_single.  You need to pass
>> the page to dma_map_page, and explicitly mange cache consistency
>> using the invalidate_kernel_vmap_range and flush_kernel_vmap_range
>> helpers.
> Please correct me if I am wrong. According to my understanding, cache
> consistency could be solved inside dma_map_page via either
> dma_direct_map_page(swio/arch_sync_dma_for_device) or ops->map_page.
> The original thought of rejecting vmalloc is that this pa is not safe
> as this mapping could go in any time. What I am suggesting is to let
> this kind of va be enrolled.

No, the point is that dma_map_single() uses virt_to_page(), and 
virt_to_page() is definitely not valid for vmalloc addresses. At worst 
it may blow up in itself with an out-of-bounds dereference; at best it's 
going to return a completely bogus page pointer which may then make 
dma_map_page() fall over.

Thanks,
Robin.

> 
>   static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
>                  size_t size, enum dma_data_direction dir, unsigned long attrs)
>   {
>         /* DMA must never operate on areas that might be remapped. */
>         if (dev_WARN_ONCE(dev, is_vmalloc_addr(ptr),
>                           "rejecting DMA map of vmalloc memory\n"))
>                 return DMA_MAPPING_ERROR;
> 
>>
>