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>

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)

arch/x86/include/asm/pgtable.h

@@ -1284,6 +1284,18 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm,
 	} while (!try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte));
 }
 
+/*
+ * Note: strictly-zero compare is narrower than pte_none(), but the gap is
+ * harmless: _PAGE_DIRTY and _PAGE_ACCESSED aren't set on untouched kernel PTEs.
+ */
+static inline bool ptep_try_set(pte_t *ptep, pte_t new_pte)
+{
+	pte_t old_pte = __pte(0);
+
+	return try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte);
+}
+#define ptep_try_set ptep_try_set
+
 #define flush_tlb_fix_spurious_fault(vma, address, ptep) do { } while (0)
 
 #define  __HAVE_ARCH_PMDP_SET_ACCESS_FLAGS

include/linux/pgtable.h

@@ -1036,6 +1036,31 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
 }
 #endif
 
+#ifndef ptep_try_set
+/**
+ * ptep_try_set - atomically set an empty kernel PTE
+ * @ptep: page table entry
+ * @new_pte: value to install
+ *
+ * Atomically set *@ptep to @new_pte iff *@ptep is pte_none(). Return true on
+ * success, false if the slot was already populated or the arch has no
+ * implementation.
+ *
+ * For special kernel page tables only - never user page tables. The caller must
+ * prevent concurrent teardown of @ptep and must accept that other writers may
+ * race. Concurrent clearers must use ptep_get_and_clear() so racing accesses
+ * agree on the outcome.
+ *
+ * Architectures opt in by providing a cmpxchg-based override and defining
+ * ptep_try_set as an identity macro. The generic stub returns false, which is
+ * correct for callers that fall through to oops on failure.
+ */
+static inline bool ptep_try_set(pte_t *ptep, pte_t new_pte)
+{
+	return false;
+}
+#endif
+
 #ifndef wrprotect_ptes
 /**
  * wrprotect_ptes - Write-protect PTEs that map consecutive pages of the same