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    SubjectRe: [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@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
    >

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