[PATCH v2 1/2] mm: cma: allocate cma areas bottom-up

[Roman Gushchin <guro@fb.com>]

Currently cma areas without a fixed base are allocated close to the
end of the node. This placement is sub-optimal because of compaction:
it brings pages into the cma area. In particular, it can bring in hot
executable pages, even if there is a plenty of free memory on the
machine. This results in cma allocation failures.

Instead let's place cma areas close to the beginning of a node.
In this case the compaction will help to free cma areas, resulting
in better cma allocation success rates.

If there is enough memory let's try to allocate bottom-up starting
with 4GB to exclude any possible interference with DMA32. On smaller
machines or in a case of a failure, stick with the old behavior.

16GB vm, 2GB cma area:
With this patch:
[    0.000000] Command line: root=/dev/vda3 rootflags=subvol=/root systemd.unified_cgroup_hierarchy=1 enforcing=0 console=ttyS0,115200 hugetlb_cma=2G
[    0.002928] hugetlb_cma: reserve 2048 MiB, up to 2048 MiB per node
[    0.002930] cma: Reserved 2048 MiB at 0x0000000100000000
[    0.002931] hugetlb_cma: reserved 2048 MiB on node 0

Without this patch:
[    0.000000] Command line: root=/dev/vda3 rootflags=subvol=/root systemd.unified_cgroup_hierarchy=1 enforcing=0 console=ttyS0,115200 hugetlb_cma=2G
[    0.002930] hugetlb_cma: reserve 2048 MiB, up to 2048 MiB per node
[    0.002933] cma: Reserved 2048 MiB at 0x00000003c0000000
[    0.002934] hugetlb_cma: reserved 2048 MiB on node 0

v2:
  - switched to memblock_set_bottom_up(true), by Mike
  - start with 4GB, by Mike

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 mm/cma.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/cma.c b/mm/cma.c
index 7f415d7cda9f..21fd40c092f0 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -337,6 +337,22 @@ int __init cma_declare_contiguous_nid(phys_addr_t base,
 			limit = highmem_start;
 		}
 
+		/*
+		 * If there is enough memory, try a bottom-up allocation first.
+		 * It will place the new cma area close to the start of the node
+		 * and guarantee that the compaction is moving pages out of the
+		 * cma area and not into it.
+		 * Avoid using first 4GB to not interfere with constrained zones
+		 * like DMA/DMA32.
+		 */
+		if (!memblock_bottom_up() &&
+		    memblock_end >= SZ_4G + size) {
+			memblock_set_bottom_up(true);
+			addr = memblock_alloc_range_nid(size, alignment, SZ_4G,
+							limit, nid, true);
+			memblock_set_bottom_up(false);
+		}
+
 		if (!addr) {
 			addr = memblock_alloc_range_nid(size, alignment, base,
 					limit, nid, true);
-- 
2.26.2