Re: [PATCH v2 2/2] dma-pool: Only allocate from CMA when in same memory zone

[Christoph Hellwig <hch@lst.de>]

On Tue, Aug 04, 2020 at 11:43:15AM +0200, Nicolas Saenz Julienne wrote:
> > Second I don't see the need (and actually some harm) in preventing GFP_KERNEL
> > allocations from dipping into lower CMA areas - something that we did support
> > before 5.8 with the single pool.
> 
> My thinking is the least we pressure CMA the better, it's generally scarse, and
> it'll not grow as the atomic pools grow. As far as harm is concerned, we now
> check addresses for correctness, so we shouldn't run into problems.
> 
> There is a potential case for architectures defining a default CMA but not
> defining DMA zones where this could be problematic. But isn't that just plain
> abusing CMA? If you need low memory allocations, you should be defining DMA
> zones.

The latter is pretty much what I expect, as we only support the default and
per-device DMA CMAs.