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b=LuvD5wtlvmMDVWxQCRAKGP5Fk5Ua91zWTtmgGiEjyZl0naJm7QD2bq1IQErR3g4kx 5g1lq7n1S4JCN4MRC6H+pQfs6eWEWioGavXdi6/0+bma0C5rW5da6/fu/4xwuJvgKe HzrTO4ttBCMkPgmdrFGCy/9oPvg6teWpN+L63Kw6eKG2HVCgK98SQ+ophIaIHfzCGU iqfBrIBYurBTrCX4cmqNgI4Boz6FhWb39R+bblyb+UeqldQesf5cc1GFb79FuOhPzP /+UhXK5vXox6n9KKV/D05ZUh267keaN2trcOykJqJLq99QW2h1NuPfnmYaqWvYKGCV 0pIzPn9odMx9w== Date: Sun, 14 Jun 2026 10:28:02 +0100 From: Will Deacon To: Tejun Heo Cc: David Vernet , Andrea Righi , Changwoo Min , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Kumar Kartikeya Dwivedi , Peter Zijlstra , Catalin Marinas , 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 Subject: Re: [PATCH 1/8] mm: Add ptep_try_set() for lockless empty-slot installs Message-ID: References: <20260522172219.1423324-1-tj@kernel.org> <20260522172219.1423324-2-tj@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260522172219.1423324-2-tj@kernel.org> X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: ACC9B140002 X-Stat-Signature: 1rkfqmmycc6pum6wftjmi6niy1fjpjjm X-Rspam-User: X-HE-Tag: 1781429291-623732 X-HE-Meta: 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 GzRF8hl5 z1BD6SQGqMguvzOYxqZlX86+FOndyxU9ldJPNmAnulCpN9qKLWDJVaBqeDznscCvFYeeNxcfCeLFN8wkTllLOhwnwXjv6b0fjPFiF98hpFIKeJLOeplYIcYMBgVfCHD8uH4OhM5K/8ewj57hGrPUSCXoiDyGoC6uVt9lM7ourOO4BEXpe/Pr/tc1Hu/cc+D9iL2SJLKkWo2pYZW6BezXFIA89D/C1OyNhTSUeX6UkziAyKMHRmHGKHwzq5zWu4550UjJUfdY1TNAsgU38lKOIfmEr9NEEDjZzcMMMWVe4OCRteO5WbJP25xDmp9Qem0JEhfUDzMS/BwvN7WzoajZLac3MdRlcYodf8qtEWqg7F0KzL0MpfwWCC/dODA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, May 22, 2026 at 07:22:12AM -1000, Tejun Heo wrote: > Add ptep_try_set(ptep, new_pte): atomically set *ptep to new_pte iff it is > currently pte_none(). Returns true on success, false if the slot was already > populated or the arch has no implementation. > > The intended caller is the upcoming bpf_arena kernel-side fault recovery > path. The install runs from a page fault that can be nested under locks > held by the faulting kernel caller (e.g. a BPF program holding > raw_res_spin_lock_irqsave on its arena's spinlock), so trylock-and-retry > would A-A deadlock. Lock-free cmpxchg is the only viable option, which > constrains this helper to special kernel page tables where concurrent > writers cooperate via atomic accessors. > > The generic version in returns false. x86 and arm64 > override with try_cmpxchg-based implementations on the underlying pteval. > Other architectures get the false stub - the callers there already fall > through to oops. > > v2: Rename to ptep_try_set(). Tighten kerneldoc. (David, Alexei) > v3: Note that strict-zero cmpxchg is narrower than pte_none(). (Andrea) > > Suggested-by: Kumar Kartikeya Dwivedi > Suggested-by: Alexei Starovoitov > Signed-off-by: Tejun Heo > Reviewed-by: Andrea Righi > Cc: David Hildenbrand > --- > arch/arm64/include/asm/pgtable.h | 12 ++++++++++++ > arch/x86/include/asm/pgtable.h | 12 ++++++++++++ > include/linux/pgtable.h | 25 +++++++++++++++++++++++++ > 3 files changed, 49 insertions(+) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 9029b81ccbe8..28bada97d443 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -1830,6 +1830,18 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm, > return __ptep_get_and_clear(mm, addr, ptep); > } > > +/* > + * Note: strictly-zero compare is narrower than pte_none(), but the gap is > + * harmless: a fresh kernel PTE has no software bits set. > + */ This comment really confused me :/ What is a "fresh" kernel PTE and why do you specifically call out "software bits" if the CAS requires all 64 bits to be 0? Why is that narrower than pte_none() given that pte_none() for arm64 is: #define pte_none(pte) (!pte_val(pte)) Will