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 <g...@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
