Re: [PATCH v2 09/26] mm/fbatch: restore mlock+munlock batching, without extra ref

Update mlock_folio(), munlock_folio() and their fbatch callouts and
helpers, to do folio_try_get()s at batch processing time, instead of
holding a folio reference all the while in mlock_fbatch: as in folio.c.

But more interesting is the use of mod_mlock_count(), using try_cmpxchg()
to update folio->mlock_count safely when possible (now when on lru_add
fbatch as well as when unevictable). While __mlock_folio() is as hard to
think about as before, __munlock_folio() simpler because munlock_folio()
can adjust mlock_count itself without clear_lru() or lruvec lock, and so
do the folio_test_clear_mlocked() immediately for itself (without which
unevictable_pgs_cleared was likely to appear high, when it should be 0
or low to indicate good mlock health).

__munlock_folio() is safe for use even when the unreferenced folio has
been freed and reused. It appears that __mlock_folio() could affect a
folio which has been freed and reused, but only if it is reused as an
mlocked folio, in which case its mlock_count is spuriously incremented
(but usually a spurious munlock decrement will follow).  How grave is
this? If unevictable_pgs_cleared remains low, not so bad.

I've gone back and forth on whether to move mlock_fbatch and these
functions into mm/folio.c: for now they stay here in mm/mlock.c.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/mlock.c | 147 +++++++++++++++++++++++++++++++----------------------
 1 file changed, 86 insertions(+), 61 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 2c690f18031e..1050010bbe0b 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c

@@ -58,6 +58,20 @@ EXPORT_SYMBOL(can_do_mlock);
  * indicate the unevictable state.
  */
 
+static long mod_mlock_count(struct folio *folio, long incdec)
+{
+	long mlock_count = READ_ONCE(folio->mlock_count);
+
+	while (mlock_count & MLOCK_COUNT_0) {
+		if (mlock_count + incdec < MLOCK_COUNT_0)
+			return MLOCK_COUNT_0;
+		if (try_cmpxchg(&folio->mlock_count, &mlock_count,
+				mlock_count + incdec))
+			return mlock_count + incdec;
+	}