[RFC PATCH v12 based on mm-unstable as of Feb 21, 2025 07/25] mm: introduce luf_ugen to be used as a global timestamp

[Byungchul Park <byungchul@sk.com>]

Functionally, no change.  This is a preparation for luf mechanism that
needs to evaluate the temporal sequence of events to determine whether
tlb flush required has been done on each CPU.

To achieve that, this patch introduced a generation number, luf_ugen,
and a few APIs manipulating the number.  It's worth noting the number is
designed to wraparound so care must be taken when using it.

Signed-off-by: Byungchul Park <byungchul@sk.com>
---
 include/linux/mm.h | 34 ++++++++++++++++++++++++++++++++++
 mm/rmap.c          | 22 ++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d82feabbe44f8..74a37cb132caa 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4240,4 +4240,38 @@ int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *st
 int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status);
 int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
 
+#if defined(CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH)
+/*
+ * luf_ugen will start with 2 so that 1 can be regarded as a passed one.
+ */
+#define LUF_UGEN_INIT 2
+
+static inline bool ugen_before(unsigned long a, unsigned long b)
+{
+	/*
+	 * Consider wraparound.
+	 */
+	return (long)(a - b) < 0;
+}
+
+static inline unsigned long next_ugen(unsigned long ugen)
+{
+	if (ugen + 1)
+		return ugen + 1;
+	/*
+	 * Avoid invalid ugen, zero.
+	 */
+	return ugen + 2;
+}
+
+static inline unsigned long prev_ugen(unsigned long ugen)
+{
+	if (ugen - 1)
+		return ugen - 1;
+	/*
+	 * Avoid invalid ugen, zero.
+	 */
+	return ugen - 2;
+}
+#endif
 #endif /* _LINUX_MM_H */
diff --git a/mm/rmap.c b/mm/rmap.c
index 546b7a6a30a44..8439dbb194c8c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -634,6 +634,28 @@ struct anon_vma *folio_lock_anon_vma_read(const struct folio *folio,
 }
 
 #ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
+
+/*
+ * This generation number is primarily used as a global timestamp to
+ * determine whether tlb flush required has been done on each CPU.  The
+ * function, ugen_before(), should be used to evaluate the temporal
+ * sequence of events because the number is designed to wraparound.
+ */
+static atomic_long_t __maybe_unused luf_ugen = ATOMIC_LONG_INIT(LUF_UGEN_INIT);
+
+/*
+ * Don't return invalid luf_ugen, zero.
+ */
+static unsigned long __maybe_unused new_luf_ugen(void)
+{
+	unsigned long ugen = atomic_long_inc_return(&luf_ugen);
+
+	if (!ugen)
+		ugen = atomic_long_inc_return(&luf_ugen);
+
+	return ugen;
+}
+
 /*
  * Flush TLB entries for recently unmapped pages from remote CPUs. It is
  * important if a PTE was dirty when it was unmapped that it's flushed
-- 
2.17.1