KVM: PPC: Book3S HV: Reduce default CMA pool size

We have observed that on machines with all their memory in a single
node, it is possible to hit an out of memory situation where kernel
allocations (which can't use the CMA pool) fail, triggering the OOM
killer, yet reclaim doesn't start because there is still free memory
in the CMA pool.  To alleviate this situation somewhat, this reduces
the default CMA pool size from 5% to 3% of system memory.  The 3%
should still be enough in most situations, and if not, the user can
specify a different amount on the kernel command line.

This should help with BZ 110181.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/powerpc/kvm/book3s_hv_builtin.c b/arch/powerpc/kvm/book3s_hv_builtin.c
index 4532996b561d18..1255dc8c98030b 100644
--- a/arch/powerpc/kvm/book3s_hv_builtin.c
+++ b/arch/powerpc/kvm/book3s_hv_builtin.c
@@ -29,9 +29,12 @@
  */
 #define HPT_ALIGN_PAGES		((1 << 18) >> PAGE_SHIFT) /* 256k */
 /*
- * By default we reserve 5% of memory for hash pagetable allocation.
+ * By default we reserve 3% of memory for hash pagetable allocation.
+ * Each guest's HPT will be sized at between 1/128 and 1/64 of its
+ * memory, i.e. up to 1.56%, and allowing for about a 2x memory
+ * overcommit factor gets us to about 3%.
  */
-static unsigned long kvm_cma_resv_ratio = 5;
+static unsigned long kvm_cma_resv_ratio = 3;
 /*
  * We allocate RMAs (real mode areas) for KVM guests from the KVM CMA area.
  * Each RMA has to be physically contiguous and of a size that the