Re: [PATCH v4] slob: add size header to all allocations

[Christoph Lameter]

On Mon, 29 Nov 2021, Rustam Kovhaev wrote:

> > I'm not sure that helps.
> > The header can't share a cache line with the previous item (because it
> > might be mapped for DMA) so will always take a full cache line.
>
> I thought that DMA API allocates buffers that are larger than page size.
> DMA pool seems to be able to give out smaller buffers, but underneath it
> seems to be calling page allocator.
> The SLOB objects that have this header are all less than page size, and
> they cannot end up in DMA code paths, or can they?

kmalloc slab allocations must return dmaable memory. If the underlying
hardware can only dma to a cache line border then all objects must be
aligned that way.