linux-kernel - Re: [PATCH v2 RFC/RFT 1/5] ARM: dma-mapping: Add fallback normal page allocations

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Message-ID: <20190424150638.GA22191@lst.de>
Date:   Wed, 24 Apr 2019 17:06:38 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     hch@....de, robin.murphy@....com, vdumpa@...dia.com,
        linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, joro@...tes.org, m.szyprowski@...sung.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, tony@...mide.com
Subject: Re: [PATCH v2 RFC/RFT 1/5] ARM: dma-mapping: Add fallback normal
 page allocations

On Tue, Mar 26, 2019 at 04:01:27PM -0700, Nicolin Chen wrote:
>  	page = dma_alloc_from_contiguous(dev, count, order, gfp & __GFP_NOWARN);
> +	if (!page)
> +		page = alloc_pages(gfp, order);

We have this fallback in most callers already.  And with me adding
it to the dma-iommu code in one series, and you to arm here I think
we really need to take a step back and think of a better way
to handle this, and the general mess that dma_alloc_from_contiguous.

So what about:

 (1) change the dma_alloc_from_contiguous prototype to be:

struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp);

     that is: calculate order and count internally, pass the full gfp_t
     and mask it internally, and drop the pointless from in the name.
     I'd also use the oppurtunity to forbid a NULL dev argument and
     opencode those uses.
 
 (2) handle the alloc_pages fallback internally.  Note that we should
     use alloc_pages_node as we do in dma-direct.

> +			if (!dma_release_from_contiguous(dev, page, count))
> +				__free_pages(page, get_order(size));

Same for dma_release_from_contiguous - drop the _from, pass the
actual size, and handle the free_pages fallback.