dma-contiguous: use fallback alloc_pages for single pages

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>
Tested-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

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)) {