[PATCH 1/8] mm: Add ptep_try_set() for lockless empty-slot installs

[Tejun Heo]

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 for kernel-PTE use.
    (David, Alexei)

Suggested-by: Kumar Kartikeya Dwivedi <memxor at gmail.com>
Suggested-by: Alexei Starovoitov <ast at kernel.org>
Signed-off-by: Tejun Heo <tj at kernel.org>
Cc: David Hildenbrand <david at kernel.org>
---
 arch/arm64/include/asm/pgtable.h |  8 ++++++++
 arch/x86/include/asm/pgtable.h   |  8 ++++++++
 include/linux/pgtable.h          | 26 ++++++++++++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 9029b81ccbe8..a129be91ef2c 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1830,6 +1830,14 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 	return __ptep_get_and_clear(mm, addr, ptep);
 }
 
+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)
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 13e3e9a054cb..047e273a4eab 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1284,6 +1284,14 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm,
 	} while (!try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte));
 }
 
+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
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index cdd68ed3ae1a..d68374f404c1 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1036,6 +1036,32 @@ 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
-- 
2.54.0