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b=BO85kgePRyguLqXdsSmYSPPbc2M9M1R9D7u7yNKftG8vO83z5DOcpC1hVIDZvumcT QWxKTrqcHbY4mwtZyZS6Ri4wQ8M2wE5vozWmFRthBrdQTSjixI1eNMVZe401kMXp9x VKQXxqMh1mY8BQveioCajwIz3qEgPD3Wz7WFV799FuuXrDcApB+DKzZxCKjOMBtPc8 xHIQ50diUzeyAWDTfME/K/4V9hozcFX8TbJXdud7+hlk9p/n3YoI3H2L1udoyJNVo2 5aUAa1AzYtzcKeTBeKIaIbwzo5WR4sOI3s9svJydmcCnk5t2Y6uhwHfkazwN33Ed9k vzcg2+4l1sZBA== Date: Fri, 29 May 2026 08:44:05 -1000 From: Tejun Heo To: Alexei Starovoitov Cc: David Hildenbrand , David Vernet , Andrea Righi , Changwoo Min , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Kumar Kartikeya Dwivedi , Peter Zijlstra , Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Andrew Morton , Mike Rapoport , Emil Tsalapatis , sched-ext@lists.linux.dev, bpf , X86 ML , linux-arm-kernel , linux-mm , LKML Subject: Re: [PATCH 2/8] bpf: Recover arena kernel faults with scratch page Message-ID: References: <20260522172219.1423324-1-tj@kernel.org> <20260522172219.1423324-3-tj@kernel.org> <7fd673df-22f3-4d70-a779-ea0b878188b3@kernel.org> <3901fe0537edee9d7acdfd91695ead28@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: jhmg5s11fw194gaz4d119nnkghrqhf6i X-Rspamd-Queue-Id: 10718100003 X-Rspamd-Server: rspam07 X-Rspam-User: X-HE-Tag: 1780080247-13872 X-HE-Meta: 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 c0sjl0Vm eu/rgHpkrsA2JgtBlnVX1n1sBEEON3a6hnYLNRBmSEayhLGQSJs8fnfAyHg7t6HYspXKWvWcLE1OMDPNaQtsml47MzlbciUnzA6amtEBaJ4D5Lbrq3ydjlXmsO/haQxDlP71ownC4tvsDmvNP7AGe8HbBSASeYHGafmxDeVLZIwgkx+tumn4HAtXy4cocgvjJi/gnxhyDvqEZA+3fnpnvKSNpTLjP4sReoS9Ap9ZV4KIrcuGzJkbEStUPQIjw5CgVHc0dpqthUvFjceKewyRkBvijFsTcqG/+ry4M Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello, On Fri, May 29, 2026 at 11:38:21AM -0700, Alexei Starovoitov wrote: ... > > 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. > > returning EBUSY because apply_range_set_cb() hit scratch page > is SEGV out of arena_vm_fault() and arguably ok-ish, > but bpf_arena_alloc_pages() returning NULL because scratch page > was in the range just sucks. > Earlier bpf prog passed the wrong arena addr to kfunc and triggered > that scratch page. It broke the contract and kept the pieces, > so ok-ish too, but overwriting scratch page with proper page > during bpf_arena_alloc_pages() is imo much better behavior. > That scratch page will cause all future bpf_arena_alloc_pages() fail as well. > Hence I prefer that check removed. Yeah, let's do that. David, would that be enough? Or are you still concerned about set_pte_at() competing with ptep_try_set()? Thanks. -- tejun