Re: [RFC v1 3/4] swiotlb: Allow dynamic allocation of bounce buffers

[Christoph Hellwig <hch@lst.de>]

On Tue, Mar 28, 2023 at 09:54:35AM +0200, Petr Tesarik wrote:
> I tend to agree here. However, it's the DMABUF design itself that causes
> some trouble. The buffer is allocated by the v3d driver, which does not
> have the restriction, so the DMA API typically allocates an address
> somewhere near the 4G boundary. Userspace then exports the buffer, sends
> it to another process as a file descriptor and imports it into the vc4
> driver, which requires DMA below 1G. In the beginning, v3d had no idea
> that the buffer would be exported to userspace, much less that it would
> be later imported into vc4.

Then we need to either:

 a) figure out a way to communicate these addressing limitations
 b) find a way to migrate a buffer into other memory, similar to
    how page migration works for page cache

> BTW my testing also suggests that the streaming DMA API is quite
> inefficient, because UAS performance _improved_ with swiotlb=force.
> Sure, this should probably be addressed in the UAS and/or xHCI driver,
> but what I mean is that moving away from swiotlb may even cause
> performance regressions, which is counter-intuitive. At least I would
> _not_ have expected it.

That is indeed very odd.  Are you running with a very slow iommu
driver there?   Or what is the actual use case there in general?

> >> +	gfp = (attrs & DMA_ATTR_MAY_SLEEP) ? GFP_KERNEL : GFP_NOWAIT;
> >> +	slot = kmalloc(sizeof(*slot), gfp | __GFP_NOWARN);
> >> +	if (!slot)
> >> +		goto err;
> >> +
> >> +	slot->orig_addr = orig_addr;
> >> +	slot->alloc_size = alloc_size;
> >> +	slot->page = dma_direct_alloc_pages(dev, PAGE_ALIGN(alloc_size),
> >> +					    &slot->dma_addr, dir,
> >> +					    gfp | __GFP_NOWARN);
> >> +	if (!slot->page)
> >> +		goto err_free_slot;
> > 
> > Without GFP_NOIO allocations this will deadlock eventually.
> 
> Ah, that would affect the non-sleeping case (GFP_KERNEL), right?

Yes.