linux-kernel - Re: [PATCH v3 2/2] dma-contiguous: Use fallback alloc

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Date:   Fri, 24 May 2019 09:16:19 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     hch@....de, robin.murphy@....com, m.szyprowski@...sung.com,
        vdumpa@...dia.com, linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, chris@...kel.net, jcmvbkbc@...il.com,
        joro@...tes.org, dwmw2@...radead.org, tony@...mide.com,
        akpm@...ux-foundation.org, sfr@...b.auug.org.au,
        treding@...dia.com, keescook@...omium.org, iamjoonsoo.kim@....com,
        wsa+renesas@...g-engineering.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, iommu@...ts.linux-foundation.org,
        dann.frazier@...onical.com
Subject: Re: [PATCH v3 2/2] dma-contiguous: Use fallback alloc_pages for
 single pages

On Thu, May 23, 2019 at 09:06:33PM -0700, Nicolin Chen wrote:
> The addresses within a single page are always contiguous, so it's
> not so necessary to always allocate one single page from CMA area.
> Since the CMA area has a limited predefined size of space, it may
> run out of space in heavy use cases, where there might be quite a
> lot CMA pages being allocated for single pages.
> 
> However, there is also a concern that a device might care where a
> page comes from -- it might expect the page from CMA area and act
> differently if the page doesn't.

How does a device know, after this call, if a CMA area was used?  From the
patches I figured a device should not care.

> 
> This patch tries to use the fallback alloc_pages path, instead of
> one-page size allocations from the global CMA area in case that a
> device does not have its own CMA area. This'd save resources from
> the CMA global area for more CMA allocations, and also reduce CMA
> fragmentations resulted from trivial allocations.
> 
> Signed-off-by: Nicolin Chen <nicoleotsuka@...il.com>
> ---
>  kernel/dma/contiguous.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index 21f39a6cb04f..6914b92d5c88 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -223,14 +223,23 @@ bool dma_release_from_contiguous(struct device *dev, struct page *pages,
>   * This function allocates contiguous memory buffer for specified device. It
>   * first tries to use device specific contiguous memory area if available or
>   * the default global one, then tries a fallback allocation of normal pages.
> + *
> + * Note that it byapss one-page size of allocations from the global area as
> + * the addresses within one page are always contiguous, so there is no need
> + * to waste CMA pages for that kind; it also helps reduce fragmentations.
>   */
>  struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
>  {
>  	int node = dev ? dev_to_node(dev) : NUMA_NO_NODE;
>  	size_t count = PAGE_ALIGN(size) >> PAGE_SHIFT;
>  	size_t align = get_order(PAGE_ALIGN(size));
> -	struct cma *cma = dev_get_cma_area(dev);
>  	struct page *page = NULL;
> +	struct cma *cma = NULL;
> +
> +	if (dev && dev->cma_area)
> +		cma = dev->cma_area;
> +	else if (count > 1)
> +		cma = dma_contiguous_default_area;

Doesn't dev_get_dma_area() already do this?

Ira

>  
>  	/* CMA can be used only in the context which permits sleeping */
>  	if (cma && gfpflags_allow_blocking(gfp)) {
> -- 
> 2.17.1
>