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

[Jeremy Linton]

On 5/14/20 6:14 AM, Christoph Hellwig wrote:
> 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_*.

No, your right, I noticed/remembered the usb_alloc vs dma_alloc 
difference right after sending that email, and was just about to say so.

 Sorry, you right.

> > 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.