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Sun, 31 May 2026 17:16:30 -0700 (PDT) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Sun, 31 May 2026 17:16:27 -0700 Message-Id: Cc: "David Vernet" , "Andrea Righi" , "Changwoo Min" , "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" , , , , , , Subject: Re: [PATCH 2/8] bpf: Recover arena kernel faults with scratch page From: "Alexei Starovoitov" To: "Tejun Heo" , "Alexei Starovoitov" , "David Hildenbrand" X-Mailer: aerc References: <20260522172219.1423324-1-tj@kernel.org> <20260522172219.1423324-3-tj@kernel.org> <7fd673df-22f3-4d70-a779-ea0b878188b3@kernel.org> <3901fe0537edee9d7acdfd91695ead28@kernel.org> <8cc56c7a4aa29628b7d17d85be7eadb9@kernel.org> In-Reply-To: <8cc56c7a4aa29628b7d17d85be7eadb9@kernel.org> On Sun May 31, 2026 at 10:47 AM PDT, Tejun Heo wrote: > Hello, > > I posted the check removal [1], and Sashiko's review flagged a > break-before-make problem with it [2] that I think is real. ohh. interesting. > So instead of just dropping the check, the install should route through a= n > invalid entry rather than overwrite in place: > > while (!ptep_try_set(pte, mk_pte(page, PAGE_KERNEL))) { > old =3D ptep_get(pte); > if (pte_none(old)) > continue; > if (WARN_ON_ONCE(pte_page(old) !=3D arena->scratch_page)) > return -EBUSY; > ptep_get_and_clear(&init_mm, addr, pte); > broke_scratch =3D true; > } > > ptep_try_set() only fills a none slot, so the slot goes scratch->none->pa= ge > and never valid->valid, and the loop copes with a concurrent fault > re-scratching it. This also closes the set_pte_at()-vs-ptep_try_set() rac= e > I raised earlier, since both sides are now cmpxchg. A broken scratch entr= y > was live, so the caller flush_tlb_kernel_range()s those pages when > broke_scratch is set, like arena_free_pages() already does after clearing= . Makes sense to me.