[PATCH v2 6/6] mm/mglru: fix potential generation folio number leak

Each generation of MGLRU accounts anon and file folio numbers
separately, and the page table walker updates each generation's
counters in batch once the walk is done. The walker promotes a
folio's generation with a cmpxchg on folio->flags, and
update_batch_size() then reads the live flags again to pick the
anon/file column to charge. The walk holds neither the lruvec lock nor
the folio lock, so the type can flip between the cmpxchg and that
read: the lazyfree path clears PG_swapbacked, and reclaim sets it back
on a dirty lazyfree folio. The batched delta pair is then recorded in
the wrong type column. Nothing reconciles it afterwards, permanently
skewing lrugen->nr_pages and the reclaim budgets derived from it.

Fix it by capturing the type from the flags snapshot the cmpxchg
linearized against: folio_update_gen() returns the type of the state
it transitioned from, and update_batch_size() accounts with that.

A folio's type only changes while it is off the LRU list, inside a
del/add pair under the lruvec lock, with the gen bits cleared in
between. The generation and PG_swapbacked sit in the same
folio->flags word, so the cmpxchg snapshot captures them together.
Let G be the generation that snapshot captured (old_gen) and G' the
one it wrote (new_gen); the CAS can land in only three places:

  - before the del: the folio is anon at G; the batch records anon
    G -> G', and the del later removes the folio from the anon
    counters;
  - between del and add: gen == -1, so folio_update_gen() returns -1
    without touching the flags and no batch is recorded; the del/add
    pair accounts for the move alone;
  - after the add: the folio is file at the fresh generation the add
    charged; the batch records file, that gen -> G', matching that
    charge.

Unlike the drift of lazy promotions, which sort_folio() repairs under
the lruvec lock, the phantom deltas from before this fix land in a
column the folio never occupies again, so nothing ever repairs them.

Fixes: 018ee47f1489 ("mm: multi-gen LRU: exploit locality in rmap")
Signed-off-by: Kairui Song <[email protected]>
---
 include/linux/mm_inline.h |  7 ++++++-
 mm/vmscan.c               | 13 +++++++------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 7f91a89b5ba3..8cf82989ec26 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -10,6 +10,11 @@
 #include <linux/userfaultfd_k.h>
 #include <linux/leafops.h>
 
+static inline int folio_flags_is_file_lru(const unsigned long *flags)
+{
+	return !test_bit(PG_swapbacked, flags);
+}
+
 /**
  * folio_is_file_lru - Should the folio be on a file LRU or anon LRU?
  * @folio: The folio to test.
@@ -27,7 +32,7 @@
  */
 static inline int folio_is_file_lru(const struct folio *folio)
 {
-	return !folio_test_swapbacked(folio);
+	return folio_flags_is_file_lru(const_folio_flags(folio, 0));
 }
 
 static __always_inline void __update_lru_size(struct lruvec *lruvec,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f8e291968342..a1497ad61ad7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3269,7 +3269,8 @@ static bool positive_ctrl_err(struct ctrl_pos *sp, struct ctrl_pos *pv)
  ******************************************************************************/
 
 /* promote pages accessed through page tables */
-static int folio_update_gen(struct folio *folio, int new_gen, const vma_flags_t *vma_flags)
+static int folio_update_gen(struct folio *folio, int new_gen, int *is_file,
+			    const vma_flags_t *vma_flags)
 {
 	unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
 	int old_gen;
@@ -3298,6 +3299,7 @@ static int folio_update_gen(struct folio *folio, int new_gen, const vma_flags_t
 		new_flags |= BIT(PG_workingset);
 	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
 
+	*is_file = folio_flags_is_file_lru(&old_flags);
 	return old_gen;
 }
 
@@ -3328,9 +3330,8 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
 }
 
 static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio,
-			      int old_gen, int new_gen)
+			      int old_gen, int new_gen, int type)
 {
-	int type = folio_is_file_lru(folio);
 	int zone = folio_zonenum(folio);
 	int delta = folio_nr_pages(folio);
 
@@ -3519,7 +3520,7 @@ static bool suitable_to_scan(int total, int young)
 static void walk_update_folio(struct lru_gen_mm_walk *walk, struct vm_area_struct *vma,
 		struct lruvec *lruvec, struct folio *folio, bool dirty)
 {
-	int new_gen, old_gen;
+	int new_gen, old_gen, file;
 
 	if (!folio)
 		return;
@@ -3532,9 +3533,9 @@ static void walk_update_folio(struct lru_gen_mm_walk *walk, struct vm_area_struc
 		folio_mark_dirty(folio);
 
 	if (walk) {
-		old_gen = folio_update_gen(folio, new_gen, &vma->flags);
+		old_gen = folio_update_gen(folio, new_gen, &file, &vma->flags);
 		if (old_gen >= 0 && old_gen != new_gen)
-			update_batch_size(walk, folio, old_gen, new_gen);
+			update_batch_size(walk, folio, old_gen, new_gen, file);
 	} else if (lru_gen_set_refs(folio, &vma->flags)) {
 		old_gen = folio_lru_gen(folio);
 		if (old_gen >= 0 && old_gen != new_gen)

-- 
2.55.0