[PATCH v4 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)
---
Applies on 7-0-rc1.

Changes since v3->v4:
 - Sashiko noticed: https://sashiko.dev/#/patchset/20260424130157.3163009-1-dev.jain%40arm.com
   Fix this by honouring vmalloc skip via GFP_SKIP_KASAN only in HW tags case,
   to avoid unintended skipping in SW/generic KASAN.
 - Instead of removing and adding GFP_SKIP_KASAN into gfp_flags, simply
   call __get_vm_area_node() without it
 - Update GFP_SKIP_KASAN documentation
 - Put missing SOB by me

v2->v3:
- Directly skip kasan_unpoison_vmalloc() for GFP_SKIP_KASAN in patch 1

v1->v2:
- 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/gfp_types.h     |  6 +++---
 include/linux/thread_info.h   |  2 +-
 kernel/fork.c                 |  5 +++--
 mm/vmalloc.c                  | 13 +++++++++----
 5 files changed, 17 insertions(+), 11 deletions(-)

-- 
2.34.1