Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

Catalin Marinas
2022-04-16T09:42:17Z

On Thu, Apr 14, 2022 at 03:25:59PM -0700, Linus Torvalds wrote:
> On Thu, Apr 14, 2022 at 12:49 PM Catalin Marinas
> <catalin.marinas@arm.com> wrote:
> > It's a lot worse, ARCH_KMALLOC_MINALIGN is currently 128 bytes on arm64.
> > I want to at least get it down to 64 with this series while preserving
> > the current kmalloc() semantics.
>
> So here's a thought - maybe we could do the reverse of GFP_DMA, and
> add a flag to the places that want small allocations and know they
> don't need DMA?

I wonder whether that's a lot more churn than trying to identify places
where a small kmalloc()'ed buffer is passed to the DMA API. DMA into
kmalloc() buffers should be a small fraction of the total kmalloc()
uses.

For kmem_cache we have the SLAB_HWCACHE_ALIGN flag. We can add a similar
GFP_ flag as that's what we care about for DMA safety. It doesn't even
need to force the alignment to ARCH_DMA_MINALIGN but just
cache_line_size() (typically 64 on arm64 while ARCH_DMA_MINALIGN is 128
for about three platforms that have this requirement).

Functions like dma_map_single() can be made to track down the origin of
the buffer when size < cache_line_size() and warn if the slab is not
correctly aligned.

--
Catalin