From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 51F363672A9; Fri, 22 May 2026 17:22:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779470543; cv=none; b=PXMGW3XNXi0xCm4oRY6cECBgQn2YRqmQFNe8jLvClqXGS9ySuIMAdciXTTUV4u1ERcCfINlMf6ymvsdws7KsB6+gHlzU0R/YcxupkN68AJ0lnkTBrutmPyeN+285rTo6PezFaVKFYJx6mTPJCJ/5/IZShz/cHBsbCLlCbutXg7s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779470543; c=relaxed/simple; bh=gw6z1Gykt50Jg9xOLkqvpiqImJGkrvkvOKgco0UaMZs=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=k0L32Ywxa+Hs1kUiR8FFEJ2Q3JwMXNyIvY2I6apwB2SNk5lwrzoUgnVLlGSqmrCPQqFczavbqqowsYVpqVJjdH2u/4PLb81t/eOjiWMOWaEIk98sYMB5DNclRf0WwlEBWTHAdr5knucHPbY3PsRuHbeIEkspRVt2QcX4FwAkU1Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Subo8wxF; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Subo8wxF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 510551F000E9; Fri, 22 May 2026 17:22:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779470540; bh=ittPUYBhsbhDW6vpPwO5mjeXazHqCn/TQP6Y2RPQFf8=; h=From:To:Cc:Subject:Date; b=Subo8wxFI57ZYTUn7j+MflfUDw1vw1qdzw+yEmQ1zQeRRqXLAFbtBmkwHP5ijZIp0 gykqtYIO/rrl71skwA50LIA4/fMKIpOqg1swKLKdj18YuPaQoNum+2tlFRjGDD27xS f7wQrCevRm/d4yC4cQswvP2Lwnn0OdGh+nv2VdX3nWljwGkZamES2DutuUJc4vQASF JIsJkjoR24hWXH+Uvqy5v7B9Al4G2Djs7P4NxZfdosVMB8FcnzDSbsq9zjJsQMTbFM lNm20DL3Hi3nhSu1tF9m8G9y07ONmbsbfPLk0yYyhQfoIccPryxwQEOhmXL3yfQVnn LV+u6dVZ1vqkA== From: Tejun Heo To: David Vernet , Andrea Righi , Changwoo Min , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Kumar Kartikeya Dwivedi Cc: Peter Zijlstra , 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 v4 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Date: Fri, 22 May 2026 07:22:11 -1000 Message-ID: <20260522172219.1423324-1-tj@kernel.org> X-Mailer: git-send-email 2.54.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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. v4: - Patch 1: note that the strict-zero cmpxchg is narrower than pte_none() in inline comments on both x86 and arm64. (Andrea) - Patch 2: stub bpf_arena_handle_page_fault() for !CONFIG_BPF_SYSCALL via a new include/linux/bpf_defs.h. (lkp) - Patch 7: scx_arena_alloc() retries via a loop instead of a single retry on pool growth. (Andrea) - Picked up Reviewed-by tags from Emil and Andrea. v3: https://lore.kernel.org/r/20260520235052.4180316-1-tj@kernel.org 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 (f31e89a8f583) Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git arena-direct-v4 Documentation/bpf/kfuncs.rst | 14 +++ arch/arm64/include/asm/pgtable.h | 12 ++ arch/arm64/mm/fault.c | 10 +- arch/x86/include/asm/pgtable.h | 12 ++ arch/x86/mm/fault.c | 12 +- include/linux/bpf.h | 14 +++ include/linux/bpf_defs.h | 19 +++ include/linux/pgtable.h | 25 ++++ 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 | 126 ++++++++++++++++++++ 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, 630 insertions(+), 128 deletions(-) Thanks. -- tejun