Re: [PATCH v3 2/2] dma-contiguous: Use fallback alloc_pages for single pages

Ira Weiny
2019-05-24T16:15:33Z

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@gmail.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
>