[PATCH] iommu/iova: change IOVA_MAG_SIZE to 127 to save memory

[Feng Tang]

kmalloc will round up the request size to power of 2, and current
iova_magazine's size is 1032 (1024+8) bytes, so each instance
allocated will get 2048 bytes from kmalloc, causing around 1KB
waste.

And in some exstreme case, the memory wasted can trigger OOM as
reported in 2019 on a crash kernel with 256 MB memory [1].

  [    4.319253] iommu: Adding device 0000:06:00.2 to group 5
  [    4.325869] iommu: Adding device 0000:20:01.0 to group 15
  [    4.332648] iommu: Adding device 0000:20:02.0 to group 16
  [    4.338946] swapper/0 invoked oom-killer: gfp_mask=0x6040c0(GFP_KERNEL|__GFP_COMP), nodemask=(null), order=0, oom_score_adj=0
  [    4.350251] swapper/0 cpuset=/ mems_allowed=0
  [    4.354618] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.19.57.mx64.282 #1
  [    4.355612] Hardware name: Dell Inc. PowerEdge R7425/08V001, BIOS 1.9.3 06/25/2019
  [    4.355612] Call Trace:
  [    4.355612]  dump_stack+0x46/0x5b
  [    4.355612]  dump_header+0x6b/0x289
  [    4.355612]  out_of_memory+0x470/0x4c0
  [    4.355612]  __alloc_pages_nodemask+0x970/0x1030
  [    4.355612]  cache_grow_begin+0x7d/0x520
  [    4.355612]  fallback_alloc+0x148/0x200
  [    4.355612]  kmem_cache_alloc_trace+0xac/0x1f0
  [    4.355612]  init_iova_domain+0x112/0x170
  [    4.355612]  amd_iommu_domain_alloc+0x138/0x1a0
  [    4.355612]  iommu_group_get_for_dev+0xc4/0x1a0
  [    4.355612]  amd_iommu_add_device+0x13a/0x610
  [    4.355612]  add_iommu_group+0x20/0x30
  [    4.355612]  bus_for_each_dev+0x76/0xc0
  [    4.355612]  bus_set_iommu+0xb6/0xf0
  [    4.355612]  amd_iommu_init_api+0x112/0x132
  [    4.355612]  state_next+0xfb1/0x1165
  [    4.355612]  amd_iommu_init+0x1f/0x67
  [    4.355612]  pci_iommu_init+0x16/0x3f
  ...
  [    4.670295] Unreclaimable slab info:
  ...
  [    4.857565] kmalloc-2048           59164KB      59164KB

Change IOVA_MAG_SIZE from 128 to 127 to make size of 'iova_magazine'
1024 bytes so that no memory will be wasted.

[1]. https://lkml.org/lkml/2019/8/12/266

Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
 drivers/iommu/iova.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index db77aa675145b..27634ddd9b904 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -614,7 +614,12 @@ EXPORT_SYMBOL_GPL(reserve_iova);
  * dynamic size tuning described in the paper.
  */
 
-#define IOVA_MAG_SIZE 128
+/*
+ * As kmalloc's buffer size is fixed to power of 2, 127 is chosen to
+ * assure size of 'iova_magzine' to be 1024 bytes, so that no memory
+ * will be wasted.
+ */
+#define IOVA_MAG_SIZE 127
 #define MAX_GLOBAL_MAGS 32	/* magazines per bin */
 
 struct iova_magazine {
-- 
2.27.0