Re: [PATCH] kernel/dma: dma_common_find_pages returns pages for requested address

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

On 27/01/2025 5:47 pm, Dima Stepanov wrote:
> On Mon, Jan 27, 2025 at 12:50 PM Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> No, that's a bug in the caller of dma_mmap_attrs(). As the kerneldoc
>> says, cpu_addr/dma_addr/size must still represent the whole buffer as
>> returned by the allocator - any offset for the mapping itself relative
>> to the start of the buffer is expressed in vma->pgoff.
>>
>> Thanks,
>> Robin.
> 
> I see, thanks for clarification Robin. I was confused, because depending
> on the backend it will work or not work. But i believe that in this case it is
> undefined behavior. That is unfortunate to me, since the idea behind was:
> - The memory allocated once because of device/firmware
> - Different users could request a part of this memory from the kernel and
> mmap it
> 
> And i didn't want to expose this offset information to the user. Wanted to
> rely on the kernel to mmap the proper part of the buffer.

In principle I don't see why you shouldn't be able to create multiple 
files representing different parts of one large buffer - seems like the 
ops for said files would just need to be a bit cleverer, and remain 
aware of the whole buffer as well as the range of their own part within 
it. Then they can adjust vma->vm_pgoff accordingly before calling 
dma_mmap_*() (and possibly perform their own stricter bounds checking as 
well.)

Thanks,
Robin.