[PATCH v4 08/17] mm/mglru, smaps: convert to LRU refs based operations for MGLRU

[Kairui Song via B4 Relay <devnull+kasong.tencent.com@kernel.org>]

From: Kairui Song <kasong@tencent.com>

PG_referenced is only the lowest bit of the folio LRU refs count
(LRU_REFS_FLAGS) now, and MGLRU never kept it stable: promotion used to
clear it, and now it toggles as the count grows. But with the LRU
referenced count as a stable API now, we can use two helpers to provide
in-kernel users a stable and sane reading of the referenced status of
an LRU managed folio.

Add folio_check_lru_refs() and folio_clear_lru_refs(), which use the refs
count when MGLRU is enabled and PG_referenced otherwise, and switch the
smaps paths over to them. The lru_gen_enabled() test stays inside the
helpers, so its static key is not splattered over the few call sites.

Convert smaps as the first user of them. MGLRU now reports every folio
with a nonzero count, so the reading no longer flickers and clearing no
longer results in an ambiguous status.

The classical LRU case is unchanged.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 fs/proc/task_mmu.c        |  6 +++---
 include/linux/mm_inline.h | 31 +++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 81c464139f8f..55f3a35fd3af 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -888,7 +888,7 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
 
 	mss->resident += size;
 	/* Accumulate the size in pages that have been accessed. */
-	if (young || folio_test_young(folio) || folio_test_referenced(folio))
+	if (young || folio_test_young(folio) || folio_check_lru_refs(folio))
 		mss->referenced += size;
 
 	/*
@@ -1685,7 +1685,7 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
 		/* Clear accessed and referenced bits. */
 		pmdp_test_and_clear_young(vma, addr, pmd);
 		folio_test_clear_young(folio);
-		folio_clear_referenced(folio);
+		folio_clear_lru_refs(folio);
 out:
 		spin_unlock(ptl);
 		return 0;
@@ -1714,7 +1714,7 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
 		/* Clear accessed and referenced bits. */
 		ptep_test_and_clear_young(vma, addr, pte);
 		folio_test_clear_young(folio);
-		folio_clear_referenced(folio);
+		folio_clear_lru_refs(folio);
 	}
 	pte_unmap_unlock(pte - 1, ptl);
 	cond_resched();
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index effa13c63632..6c9510002754 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -479,6 +479,37 @@ static __always_inline void folio_inc_lru_refs_fast(struct folio *folio)
 	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
 }
 
+/**
+ * folio_check_lru_refs - Test if an LRU managed folio has been referenced.
+ * @folio: The folio to test.
+ *
+ * Returns true if the MGLRU refs count is nonzero, or PG_referenced is set,
+ * which is also correct for the non-MGLRU case. Kept as a separate helper,
+ * to avoid splattering the lru_gen_enabled() static key.
+ */
+static inline bool folio_check_lru_refs(const struct folio *folio)
+{
+	if (lru_gen_enabled())
+		return folio_lru_refs(folio) > 0;
+	return folio_test_referenced(folio);
+}
+
+/**
+ * folio_clear_lru_refs - Clear the referenced state of an LRU managed folio.
+ * @folio: The folio to clear.
+ *
+ * Reset the MGLRU refs count to zero, or clear PG_referenced, leaving
+ * PG_workingset alone for the non-MGLRU case. Kept as a separate helper
+ * just like folio_check_lru_refs().
+ */
+static inline void folio_clear_lru_refs(struct folio *folio)
+{
+	if (lru_gen_enabled())
+		folio_set_lru_refs(folio, 0);
+	else
+		folio_clear_referenced(folio);
+}
+
 static __always_inline
 void lruvec_add_folio(struct lruvec *lruvec, struct folio *folio)
 {

-- 
2.56.0