Re: Validating dma_mmap_coherent() parameters before calling (was Re: WARNING in memtype_reserve)

[Christoph Hellwig]

On Thu, May 14, 2020 at 06:10:03AM -0500, Jeremy Linton wrote:
>> I only need to look at the commit for 3 seconds to tell you that it is
>> completely buggy.  While using dma_mmap_coherent is fundamentally the
>> right thing and absolutely required for dma_alloc_* allocations, USB
>> also uses it's own local gen pool allocator or plain kmalloc for not
>> DMA capable controller.  This need to use remap_pfn_range.  I'm pretty
>> sure you hit one of those cases.
>
> ? The code path in question is usbdev_mmap() and the allocation is done ~13 
> lines lines before as a usb_alloc_coherent().

And did you take a look at how usb_alloc_coherent is implemented?  That
should make it completely obvious that not all allocations come
from dma_alloc_*.

> That sort of makes sense, except for the above, and the fact that I would 
> imagine the dma_mmap_coherent should be dealing with that case. I'm not 
> really clear about the details of the GCE usb device here, but my first 
> guess at this was the dma_pgprot() in dma_direct_mmap() is incorrectly 
> picking a pgprot...

No, dma_mmap_* / dma_direct_mmap has absolutely no business dealing
with memory that did not come from the DMA allocator.