[PATCH v3 0/3] kasan: hw_tags: Disable tagging for stack and page-tables

[Dev Jain <dev.jain@arm.com>]

Stacks and page tables are always accessed with the match-all tag,
so assigning a new random tag every time at allocation and setting
invalid tag at deallocation time, just adds overhead without improving
the detection.

With __GFP_SKIP_KASAN the page keeps its poison tag and KASAN_TAG_KERNEL
(match-all tag) is stored in the page flags while keeping the poison tag
in the hardware. The benefit of it is that 256 tag setting instruction
per 4 kB page aren't needed at allocation and deallocation time.

Thus match-all pointers still work, while non-match tags (other than
poison tag) still fault.

__GFP_SKIP_KASAN only skips for KASAN_HW_TAGS mode, so coverage is
unchanged.

Benchmark:
The benchmark has two modes. In thread mode, the child process forks
and creates N threads. In pgtable mode, the parent maps and faults a
specified memory size and then forks repeatedly with children exiting
immediately.

Thread benchmark:
2000 iterations, 2000 threads:	2.575 s → 2.229 s (~13.4% faster)

The pgtable samples:
- 2048 MB, 2000 iters		19.08 s → 17.62 s (~7.6% faster)
---

Changes since v2:
- Directly skip kasan_unpoison_vmalloc() for GFP_SKIP_KASAN in patch 1

Changes since v1:
- Update description/title
- Patch 1: Simplify skip conditions based on the fact that __GFP_SKIP_KASAN
- Patch 2: Specify _GFP_SKIP_KASAN in THREADINFO_GFP and GFP_VMAP_STACK

Muhammad Usama Anjum (3):
  vmalloc: add __GFP_SKIP_KASAN support
  kasan: skip HW tagging for all kernel thread stacks
  mm: skip KASAN tagging for page-allocated page tables

 include/asm-generic/pgalloc.h |  2 +-
 include/linux/thread_info.h   |  2 +-
 kernel/fork.c                 |  5 +++--
 mm/vmalloc.c                  | 20 +++++++++++++++++---
 4 files changed, 22 insertions(+), 7 deletions(-)

-- 
2.34.1