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b=b1E3GALC51Qetd6cxGgeJWodhsrwgRKSs66mftcTKX7YzsuPZuhfJgvzz9CTKlQ0j RD04UYukWDAL3gqHgnYHUJSRZ1ywZiNWfWZnr4UkHIZ76W/0P/hmT2oWqo8zkGp5bl kUm5Di17xgAkIoWjni6+yWi2vr9iHQxGTZZWvtqmffmGxPcu3AE1BDpnCDA11bOruT sQ/JBJ2qLlebcnI3trMS7Ao+wiulQYCuoVdrFRBwWQURPs/2oqdaGpVEOkYX6klWLk bUcW0aEfl155cUNw+IrTf7HNrcYSpWusjR8szBAfI3Gpm19fwj2+HsBBHnICPKQVnE te8EPOPinvSRQ== From: Tejun Heo To: David Vernet , Andrea Righi , Changwoo Min , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Kumar Kartikeya Dwivedi Cc: Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Andrew Morton , David Hildenbrand , Mike Rapoport , Emil Tsalapatis , sched-ext@lists.linux.dev, bpf@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Tejun Heo Subject: [PATCHSET v2 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Date: Sun, 17 May 2026 11:12:24 -1000 Message-ID: <20260517211232.1670594-1-tj@kernel.org> X-Mailer: git-send-email 2.54.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: DE9A040008 X-Rspam-User: X-Stat-Signature: ir5nyf3a3tux4d58hre6z88pifoo1zx9 X-HE-Tag: 1779052354-276829 X-HE-Meta: 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 7YjyFYSo ntTpR8Z7U5HLoE2v9pB03pMsi5ZEyRGm4sa+2juz5V+ozmLBtAoHgupgVZ9kuctd5Am+BjSOzSJfUTbby2joTAOaD7voz0hp77XK0LZhfBaVtBZwUGPOGrpCOdEhKgP7Cm290sD+qfQfEXgg2mm0avt6r7Mk8/Vmz+0+k9cumXfAWmr6Lr1DeJrvL8grm7Ud12WSioJIIBtRR9B9ALLBADGS2DOWgHed7C4wOuJMj6VNjgOFbyWbT59ko8OI6qlO2jjuYblx1OLZkir+iuUB3OKFYgUlZa0LQiQ7t Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. v2 changes since RFC v1 (20260427105109.2554518-1-tj@kernel.org): Switched from pre-filling the arena recovery range with a fallback page to leaving PTEs sparse and installing the scratch page from an arch fault hook on demand. Pre-fill erased the fault and lost error attribution; on-demand keeps the fault visible and reports the violation through the program's bpf_stream. Plus misc fixes. 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_install() 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_install() 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 (c9017d335aab) with cmask-prep-v2.1 applied: https://lore.kernel.org/r/20260517183614.1191534-1-tj@kernel.org Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git arena-direct-v2 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 | 16 +++ kernel/bpf/arena.c | 209 ++++++++++++++++++++++++++++------ 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, 604 insertions(+), 123 deletions(-) Thanks. -- tejun