mm: Add ptep_try_set() for lockless empty-slot installs

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 <linux/pgtable.h> 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 <memxor@gmail.com>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Link: https://lore.kernel.org/r/20260522172219.1423324-2-tj@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 4dfa42b7d053..984f0502c9d0 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.
+ */
+static inline bool ptep_try_set(pte_t *ptep, pte_t new_pte)
+{
+	pteval_t old = 0;
+
+	return try_cmpxchg(&pte_val(*ptep), &old, pte_val(new_pte));
+}
+#define ptep_try_set ptep_try_set
+
 #define test_and_clear_young_ptes test_and_clear_young_ptes
 static inline bool test_and_clear_young_ptes(struct vm_area_struct *vma,
 		unsigned long addr, pte_t *ptep, unsigned int nr)