[PATCH] update to "kasan: support backing vmalloc space with real shadow memory"

Daniel Axtens
2019-11-20T05:27:29Z

Hi Andrew,

This is a quick fixup to patch 1 of the "kasan: support backing
vmalloc space with real shadow memory" series, v11, which you pulled
in to your mmotm tree.

There are 2 changes:

- A fixup to the per-cpu allocator path to avoid allocating memory
under a spinlock, thanks Qian Cai.

- Insert flush_cache_vmap() between mapping shadow and poisoning
it. This is a no-op on x86 and arm64, but on powerpc it does a
ptesync instruction which prevents occasional page faults.

Here are updated benchmark figures for the commit message:

Testing with test_vmalloc.sh on an x86 VM with 2 vCPUs shows that:

- Turning on KASAN, inline instrumentation, without vmalloc, introuduces
a 5.7x-6.4x slowdown in vmalloc operations.

- Turning this on introduces the following slowdowns over KASAN:
* ~1.82x slower single-threaded (test_vmalloc.sh performance)
* ~2.11x slower when both cpus are performing operations
simultaneously (test_vmalloc.sh sequential_test_order=1)

This is unfortunate, but given that this is a debug feature only, not
the end of the world.

The full results are:

Performance

No KASAN KASAN original x baseline KASAN vmalloc x baseline x KASAN

fix_size_alloc_test 662004 11404956 17.23 19144610 28.92 1.68
full_fit_alloc_test 710950 12029752 16.92 13184651 18.55 1.10
long_busy_list_alloc_test 9431875 43990172 4.66 82970178 8.80 1.89
random_size_alloc_test 5033626 23061762 4.58 47158834 9.37 2.04
fix_align_alloc_test 1252514 15276910 12.20 31266116 24.96 2.05
random_size_align_alloc_te 1648501 14578321 8.84 25560052 15.51 1.75
align_shift_alloc_test 147 830 5.65 5692 38.72 6.86
pcpu_alloc_test 80732 125520 1.55 140864 1.74 1.12
Total Cycles 119240774314 763211341128 6.40 1390338696894 11.66 1.82

Sequential, 2 cpus

No KASAN KASAN original x baseline KASAN vmalloc x baseline x KASAN

fix_size_alloc_test 1423150 14276550 10.03 27733022 19.49 1.94
full_fit_alloc_test 1754219 14722640 8.39 15030786 8.57 1.02
long_busy_list_alloc_test 11451858 52154973 4.55 107016027 9.34 2.05
random_size_alloc_test 5989020 26735276 4.46 68885923 11.50 2.58
fix_align_alloc_test 2050976 20166900 9.83 50491675 24.62 2.50
random_size_align_alloc_te 2858229 17971700 6.29 38730225 13.55 2.16
align_shift_alloc_test 405 6428 15.87 26253 64.82 4.08
pcpu_alloc_test 127183 151464 1.19 216263 1.70 1.43
Total Cycles 54181269392 308723699764 5.70 650772566394 12.01 2.11
fix_size_alloc_test 1420404 14289308 10.06 27790035 19.56 1.94
full_fit_alloc_test 1736145 14806234 8.53 15274301 8.80 1.03
long_busy_list_alloc_test 11404638 52270785 4.58 107550254 9.43 2.06
random_size_alloc_test 6017006 26650625 4.43 68696127 11.42 2.58
fix_align_alloc_test 2045504 20280985 9.91 50414862 24.65 2.49
random_size_align_alloc_te 2845338 17931018 6.30 38510276 13.53 2.15
align_shift_alloc_test 472 3760 7.97 9656 20.46 2.57
pcpu_alloc_test 118643 132732 1.12 146504 1.23 1.10
Total Cycles 54040011688 309102805492 5.72 651325675652 12.05 2.11

Cc: Qian Cai <cai@lca.pw>
Signed-off-by: Daniel Axtens <dja@axtens.net>
---
mm/kasan/common.c | 2 ++
mm/vmalloc.c | 5 ++++-
2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 6e7bc5d3fa83..df3371d5c572 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -794,6 +794,8 @@ int kasan_populate_vmalloc(unsigned long requested_size, struct vm_struct *area)
if (ret)
return ret;

+ flush_cache_vmap(shadow_start, shadow_end);
+
kasan_unpoison_shadow(area->addr, requested_size);

area->flags |= VM_KASAN;
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a4b950a02d0b..bf030516258c 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3417,11 +3417,14 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,

setup_vmalloc_vm_locked(vms[area], vas[area], VM_ALLOC,
pcpu_get_vm_areas);
+ }
+ spin_unlock(&vmap_area_lock);

+ /* populate the shadow space outside of the lock */
+ for (area = 0; area < nr_vms; area++) {
/* assume success here */
kasan_populate_vmalloc(sizes[area], vms[area]);
}
- spin_unlock(&vmap_area_lock);

kfree(vas);
return vms;
--
2.20.1