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

Nicolin Chen
2019-05-24T04:08:11Z

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.

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;

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