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From: Tejun Heo <tj@kernel.org>
To: David Hildenbrand <david@kernel.org>,
	David Vernet <void@manifault.com>,
	Andrea Righi <arighi@nvidia.com>,
	Changwoo Min <changwoo@igalia.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Thomas Gleixner <tglx@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Rapoport <rppt@kernel.org>,
	Emil Tsalapatis <emil@etsalapatis.com>,
	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
Subject: Re: [PATCH 2/8] bpf: Recover arena kernel faults with scratch page
Date: Fri, 29 May 2026 08:12:35 -1000	[thread overview]
Message-ID: <3901fe0537edee9d7acdfd91695ead28@kernel.org> (raw)
In-Reply-To: <DIUMP21RASZI.2VNQV45RIL209@gmail.com>

Hello,

> It should do set_pte_at() unconditionally.

I think two concerns are tangled here.

1. The racing write. set_pte_at() and the scratch installer's
ptep_try_set() hit the same PTE with no common lock. On x86-64 and arm64
set_pte_at() is a single atomic store, so it can't tear against the
cmpxchg, but a plain store racing a cmpxchg isn't atomic in general.
David, is that the worry - an arch where set_pte_at() is split and could
tear - or something else?

2. The SEGV. It's a BPF program failure propagating out as a SEGV. Maybe
not ideal, but as long as we surface the BPF error properly, it doesn't
necessarily seem broken to me.

Thanks.

--
tejun


  reply	other threads:[~2026-05-29 18:12 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-05-22 17:22 [PATCHSET v4 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Tejun Heo
2026-05-22 17:22 ` [PATCH 1/8] mm: Add ptep_try_set() for lockless empty-slot installs Tejun Heo
2026-05-22 22:07   ` David Hildenbrand (Arm)
2026-05-25 15:50   ` patchwork-bot+netdevbpf
2026-05-22 17:22 ` [PATCH 2/8] bpf: Recover arena kernel faults with scratch page Tejun Heo
2026-05-26 12:45   ` David Hildenbrand (Arm)
2026-05-28 21:30     ` Alexei Starovoitov
2026-05-29 18:12       ` Tejun Heo [this message]
2026-05-29 18:38         ` Alexei Starovoitov
2026-05-29 18:44           ` Tejun Heo
2026-05-31 17:47             ` Tejun Heo
2026-05-31 18:58               ` David Hildenbrand (Arm)
2026-06-01  0:16               ` Alexei Starovoitov
2026-05-22 17:22 ` [PATCH 3/8] bpf: Add sleepable variant of bpf_arena_alloc_pages for kernel callers Tejun Heo
2026-05-22 17:22 ` [PATCH 4/8] bpf: Add bpf_struct_ops_for_each_prog() Tejun Heo
2026-05-22 17:22 ` [PATCH 5/8] bpf/arena: Add bpf_arena_map_kern_vm_start() and bpf_prog_arena() Tejun Heo
2026-05-22 17:22 ` [PATCH 6/8] sched_ext: Require an arena for cid-form schedulers Tejun Heo
2026-05-22 17:22 ` [PATCH 7/8] sched_ext: Sub-allocator over kernel-claimed BPF arena pages Tejun Heo
2026-05-22 17:22 ` [PATCH 8/8] sched_ext: Convert ops.set_cmask() to arena-resident cmask Tejun Heo
2026-05-25 15:45 ` [PATCHSET v4 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Alexei Starovoitov
2026-05-25 19:54 ` Tejun Heo
  -- strict thread matches above, loose matches on Subject: below --
2026-05-20 23:50 [PATCHSET v3 " Tejun Heo
2026-05-20 23:50 ` [PATCH 2/8] bpf: Recover arena kernel faults with scratch page Tejun Heo
2026-05-21  3:16   ` Emil Tsalapatis
2026-05-21  9:42   ` Alexei Starovoitov
2026-05-21 17:39     ` Tejun Heo
2026-05-17 21:12 [PATCHSET v2 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Tejun Heo
2026-05-17 21:12 ` [PATCH 2/8] bpf: Recover arena kernel faults with scratch page Tejun Heo

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