Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone

[Christoph Hellwig]

On Fri, Dec 17, 2021 at 11:38:27AM +0000, Hyeonggon Yoo wrote:
> My understanding is any buffer requested from kmalloc (without
> GFP_DMA/DMA32) can be used by device driver because it allocates
> continuous physical memory. It doesn't mean that buffer allocated
> with kmalloc is free of addressing limitation.

Yes.

> 
> the addressing limitation comes from the capability of device, not
> allocation size. if you allocate memory using alloc_pages() or kmalloc(),
> the device has same limitation. and vmalloc can't be used for
> devices because they have no MMU.

vmalloc can be used as well, it just needs to be setup as a scatterlist
and needs a little lover for DMA challenged platforms with the
invalidate_kernel_vmap_range and flush_kernel_vmap_range helpers.

> But we can map memory outside DMA zone into bounce buffer (which resides
> in DMA zone) using DMA API.

Yes, although in a few specific cases the bounce buffer could also come
from somewhere else.