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

[Nicolas Saenz Julienne]

On Tue, 2020-08-04 at 08:06 +0200, Christoph Hellwig wrote:
> On Mon, Aug 03, 2020 at 06:09:56PM +0200, Nicolas Saenz Julienne wrote:
> > +	if (IS_ENABLED(CONFIG_ZONE_DMA) && (gfp & GFP_DMA))
> > +		return end <= DMA_BIT_MASK(zone_dma_bits);
> > +	if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32))
> > +		return end <= DMA_BIT_MASK(32);
> > +	if (gfp & GFP_KERNEL)
> > +		return end > DMA_BIT_MASK(32);
> 
> So the GFP_KERNEL one here looks weird.  For one I don't think the if
> line is needed at all, and it just confuses things.

Yes, sorry, shoud've seen that.

> 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.

Regards,
Nicolas

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