linux-kernel - Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve per-numa CMA

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Date:   Wed, 22 Jul 2020 16:29:43 +0200
From:   Christoph Hellwig <hch@....de>
To:     Barry Song <song.bao.hua@...ilicon.com>
Cc:     hch@....de, m.szyprowski@...sung.com, robin.murphy@....com,
        will@...nel.org, ganapatrao.kulkarni@...ium.com,
        catalin.marinas@....com, iommu@...ts.linux-foundation.org,
        linuxarm@...wei.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Steve Capper <steve.capper@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve
 per-numa CMA

On Sun, Jun 28, 2020 at 11:12:50PM +1200, Barry Song wrote:
>  struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
>  {
>  	size_t count = size >> PAGE_SHIFT;
>  	struct page *page = NULL;
>  	struct cma *cma = NULL;
> +	int nid = dev ? dev_to_node(dev) : NUMA_NO_NODE;
> +	bool alloc_from_pernuma = false;
> +
> +	if ((count <= 1) && !(dev && dev->cma_area))
> +		return NULL;
>  
>  	if (dev && dev->cma_area)
>  		cma = dev->cma_area;
> -	else if (count > 1)
> +	else if ((nid != NUMA_NO_NODE) && dma_contiguous_pernuma_area[nid]
> +		&& !(gfp & (GFP_DMA | GFP_DMA32))) {
> +		cma = dma_contiguous_pernuma_area[nid];
> +		alloc_from_pernuma = true;
> +	} else {
>  		cma = dma_contiguous_default_area;
> +	}

I find the function rather confusing now.  What about something
like (this relies on the fact that dev should never be NULL in the
DMA API)

struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
{
	size_t cma_align = min_t(size_t, get_order(size), CONFIG_CMA_ALIGNMENT);
 	size_t count = size >> PAGE_SHIFT;

	if (gfpflags_allow_blocking(gfp))
		return NULL;
	gfp &= __GFP_NOWARN;

	if (dev->cma_area)
		return cma_alloc(dev->cma_area, count, cma_align, gfp);
	if (count <= 1)
		return NULL;

	if (IS_ENABLED(CONFIG_PERNODE_CMA) && !(gfp & (GFP_DMA | GFP_DMA32)) {
		int nid = dev_to_node(dev);
 		struct cma *cma = dma_contiguous_pernuma_area[nid];
		struct page *page;

		if (cma) {
			page = cma_alloc(cma, count, cma_align, gfp);
			if (page)
				return page;
		}
	}

	return cma_alloc(dma_contiguous_default_area, count, cma_align, gfp);
}

> +		/*
> +		 * otherwise, page is from either per-numa cma or default cma
> +		 */
> +		int nid = page_to_nid(page);
> +
> +		if (nid != NUMA_NO_NODE) {
> +			if (cma_release(dma_contiguous_pernuma_area[nid], page,
> +						PAGE_ALIGN(size) >> PAGE_SHIFT))
> +				return;
> +		}
> +
> +		if (cma_release(dma_contiguous_default_area, page,

How can page_to_nid ever return NUMA_NO_NODE?