From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 CD68721A453; Sun, 17 May 2026 21:12:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779052353; cv=none; b=uNi9ysEaHvt/0ffUIK4nUB4jQos7dfqGTax2V06uZvb6UcoMXhltPSoWdDMtNTvhi2KYMXx08v44vhTa1aAGETIcAphgU5AoGi0dsSLfT/vhTKQUPjotCI6kVDH1NDkRCqOB6i0Zp6HUrH5oMABwEvBi1u+Op/6fHHArax5ZaPI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779052353; c=relaxed/simple; bh=a6e2ffrarYvyiSnMafrOD30AH1WgN2On1KkShq4mS0c=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=ZHTPCSBokEhr9wrAP66Ik+ZdSzgienM/2f+XaTT6ouFC//Ulgi1dmhKqcbs+ikdZwxlSC9yCpPxhozyWT+wJRvEhX9gVwbm1t3FY/lPOJ4sMlNNGSHFdbXRvuaxIyuNHhjK2zsjAwbz7FI86bVePh2jhIdP9c/Y0C41GTxwRlH4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=b1E3GALC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="b1E3GALC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3183CC2BCB0; Sun, 17 May 2026 21:12:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1779052353; bh=a6e2ffrarYvyiSnMafrOD30AH1WgN2On1KkShq4mS0c=; h=From:To:Cc:Subject:Date:From; 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 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. 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