Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Daniel Vetter]

On Wed, Jul 10, 2024 at 04:14:18PM +0200, Christian König wrote:
> Am 10.07.24 um 15:57 schrieb Lei Liu:
> > Use vm_insert_page to establish a mapping for the memory allocated
> > by dmabuf, thus supporting direct I/O read and write; and fix the
> > issue of incorrect memory statistics after mapping dmabuf memory.
> 
> Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.
> 
> We already discussed enforcing that in the DMA-buf framework and this patch
> probably means that we should really do that.

Last time I looked dma_mmap doesn't guarantee that the vma end sup with
VM_SPECIAL, and that's pretty much the only reason why we can't enforce
this. But we might be able to enforce this at least on some architectures,
I didn't check for that ... if at least x86-64 and arm64 could have the
check, that would be great. So might be worth it to re-audit this all.

I think all other dma-buf exporters/allocators do only create VM_SPECIAL
vmas.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch