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

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Date:   Tue, 4 Aug 2020 08:06:33 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     amit.pundir@...aro.org, hch@....de, linux-kernel@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, rientjes@...gle.com,
        jeremy.linton@....com, linux-rpi-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v2 2/2] dma-pool: Only allocate from CMA when in same
 memory zone

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.  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.