[PATCHSET v3 sched_ext/for-7.2] bpf/arena: Direct kernel-side access

[Tejun Heo <tj@kernel.org>]

Hello,

This makes BPF arena memory directly dereferenceable from kernel code
(struct_ops callbacks, kfuncs). Each arena gets a per-arena scratch page
that an arch fault hook installs into empty PTEs on kernel-side faults,
after KFENCE. The faulting instruction retries and the violation is reported
through the program's BPF stream.

v3:
- Patch 1: rename ptep_try_install() to ptep_try_set(). Tighten kerneldoc
  for kernel-PTE use. (David Hildenbrand, Alexei)
- Patch 2: apply_range_clear_cb() uses ptep_get_and_clear() so the install
  and clear sides race through atomic accessors. (David)

v2: https://lore.kernel.org/r/20260517211232.1670594-1-tj@kernel.org
v1 (RFC): https://lore.kernel.org/r/20260427105109.2554518-1-tj@kernel.org

Motivation
----------

sched_ext's ops_cid.set_cmask() hands the BPF scheduler a struct scx_cmask
*. The kernel translates a kernel cpumask to a cmask, but it had no way to
write into the arena, so the cmask lived in kernel memory and was passed as
a trusted pointer. BPF cmask helpers all operate on arena cmasks though, so
the BPF side had to word-by-word probe-read the kernel cmask into an arena
cmask via cmask_copy_from_kernel() before any helper could touch it. It
works, but is clumsy.

The shape isn't unique to set_cmask. Sub-scheduler support is on the way and
more sched_ext callbacks will want to pass structured data to BPF. Anywhere
a kfunc or struct_ops callback wants to hand a struct to a BPF program,
arena residence is the natural answer.

Approach
--------

Each arena gets a per-arena scratch page. Arenas stay sparsely mapped as
today - PTEs are populated only for allocated pages. A new arch fault hook
(bpf_arena_handle_page_fault) is wired into x86 page_fault_oops() and arm64
__do_kernel_fault(), after KFENCE. When a kernel-side access faults inside
an arena's kern_vm range, the helper walks the stack to find the BPF program
responsible, range-checks the fault address against prog->aux->arena, and
atomically installs the scratch page into the empty PTE via the new
ptep_try_set() wrapper. The kernel instruction retries and reads/writes the
scratch page. Free paths and map destruction treat scratch as non-owned.
Real allocation refuses to overwrite scratch (apply_range_set_cb returns
-EBUSY). A scratched address stays dead until map destroy, since its
presence means the BPF program has already malfunctioned.

The mechanism is default behavior - no UAPI flag.

What this preserves
-------------------

All the debugging properties of today's sparse-PTE design are preserved:

* BPF programs still fault on unmapped arena accesses. The fault semantics
  (instruction retry with rdst = 0) and the violation report through
  bpf_streams are unchanged for prog-side accesses.

* The first kernel-side touch of an unmapped address is reported via
  bpf_streams the same way as a prog-side fault, with the stack walk
  attributing it to the originating prog.

* User-side fault on a never-scratched address still lazy-allocates a real
  page (or returns SIGSEGV under BPF_F_SEGV_ON_FAULT). User-side fault on a
  scratched address SIGSEGVs.

What changes for the kernel-side caller is just that an unmapped deref no
longer oopses - it retries through the scratch page and emits a violation
report. The same shape today's BPF instruction faults have.

Patches 1-2 (atomic PTE install + arena scratch-page recovery)
--------------------------------------------------------------

  mm: Add ptep_try_set() for lockless empty-slot installs
  bpf: Recover arena kernel faults with scratch page

Patches 3-5 (helpers used by struct_ops registration)
-----------------------------------------------------

  bpf: Add sleepable variant of bpf_arena_alloc_pages for kernel callers
  bpf: Add bpf_struct_ops_for_each_prog()
  bpf/arena: Add bpf_arena_map_kern_vm_start() and bpf_prog_arena()

Patches 6-8 (sched_ext: arena auto-discovery, allocator, set_cmask)
-------------------------------------------------------------------

  sched_ext: Require an arena for cid-form schedulers
  sched_ext: Sub-allocator over kernel-claimed BPF arena pages
  sched_ext: Convert ops.set_cmask() to arena-resident cmask

Patch 6 reads each member prog's prog->aux->arena via bpf_prog_arena() and
requires the cid-form struct_ops to reference exactly one arena. Patch 7
builds a gen_pool sub-allocator inside that arena. Patch 8 converts
set_cmask() to write into arena memory; BPF dereferences via __arena like
any other arena struct, no probe-reads.

Base
----

sched_ext/for-7.2 (1136fb1213d1) with cmask-prep-v2.3 applied:
  https://lore.kernel.org/r/20260519075838.2706712-1-tj@kernel.org

Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git arena-direct-v3

 Documentation/bpf/kfuncs.rst          |  14 +++
 arch/arm64/include/asm/pgtable.h      |   8 ++
 arch/arm64/mm/fault.c                 |  10 +-
 arch/x86/include/asm/pgtable.h        |   8 ++
 arch/x86/mm/fault.c                   |  12 +-
 include/linux/bpf.h                   |  14 +++
 include/linux/bpf_defs.h              |  11 ++
 include/linux/pgtable.h               |  26 ++++
 kernel/bpf/arena.c                    | 216 +++++++++++++++++++++++++++-------
 kernel/bpf/bpf_struct_ops.c           |  36 ++++++
 kernel/bpf/core.c                     |   5 +
 kernel/sched/build_policy.c           |   4 +
 kernel/sched/ext.c                    | 135 ++++++++++++++++++++-
 kernel/sched/ext_arena.c              | 127 ++++++++++++++++++++
 kernel/sched/ext_arena.h              |  18 +++
 kernel/sched/ext_cid.c                |  20 +---
 kernel/sched/ext_internal.h           |  23 +++-
 tools/sched_ext/include/scx/cid.bpf.h |  52 --------
 tools/sched_ext/scx_qmap.bpf.c        |   5 +-
 19 files changed, 616 insertions(+), 128 deletions(-)

Thanks.

--
tejun