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

[Christoph Hellwig]

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.

> 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.