[PATCH v2 2/2] treewide: Add the __GFP_PACKED flag to several non-DMA kmalloc() allocations

[Catalin Marinas]

On Tue, Nov 01, 2022 at 06:24:16PM +0100, Christoph Hellwig wrote:
> On Tue, Nov 01, 2022 at 05:19:46PM +0000, Catalin Marinas wrote:
> > 
> > The main downside of bouncing is mobile phones where those vendors are
> > in the habit of passing noswiotlb on the kernel command line or they
> > want a very small bounce buffer (hard to pick a universally small size).
> > I guess we can go ahead with this and, depending on how bad the problem
> > is, we can look at optimising swiotlb to use a kmem_cache (or aligned
> > kmalloc) as fall-back for bouncing.
> 
> Theses phones setups are completely broken already.  There is a reason
> why it needs a debug option to disable swiotlb, and that reason is
> that Linux guarantees that the 32-bit DMA always works.  If they force
> swiotlb off they can keep the pieces as this is not a supported
> configuration.

There's also the case of low-end phones with all RAM below 4GB and arm64
doesn't allocate the swiotlb. Not sure those vendors would go with a
recent kernel anyway.

So the need for swiotlb now changes from 32-bit DMA to any DMA
(non-coherent but we can't tell upfront when booting, devices may be
initialised pretty late).

> > As a less than optimal solution, we can force bouncing for the whole
> > list if any of the sg elements is below the alignment size. Hopefully we
> > won't have many such mixed size cases.
> 
> I suspect we won't see any such cases.  The scatterlist is usually used
> for large data transfers, and many devices won't like unaligned buffers
> for SG operations to start with.  So I think it is a perfectly fine
> tradeoff.

I'll give it a try this week, hopefully post patches soon(ish).

-- 
Catalin