[PATCH v2 1/2] mm: swap: enforce updating inuse_pages at the end of swap_range_free()

[Yosry Ahmed <yosryahmed@google.com>]

In swap_range_free(), we update inuse_pages then do some cleanups (arch
invalidation, zswap invalidation, swap cache cleanups, etc). During
swapoff, try_to_unuse() checks that inuse_pages is 0 to make sure all
swap entries are freed. Make sure we only update inuse_pages after we
are done with the cleanups in swap_range_free(), and use the proper
memory barriers to enforce it. This makes sure that code following
try_to_unuse() can safely assume that swap_range_free() ran for all
entries in thr swapfile (e.g. swap cache cleanup, zswap_swapoff()).

In practice, this currently isn't a problem because swap_range_free() is
called with the swap info lock held, and the swapoff code happens to
spin for that after try_to_unuse(). However, this seems fragile and
unintentional, so make it more relable and future-proof. This also
facilitates a following simplification of zswap_swapoff().

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
---
 mm/swapfile.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index b11b6057d8b5f..0580bb3e34d77 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -737,8 +737,6 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 		if (was_full && (si->flags & SWP_WRITEOK))
 			add_to_avail_list(si);
 	}
-	atomic_long_add(nr_entries, &nr_swap_pages);
-	WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
 	if (si->flags & SWP_BLKDEV)
 		swap_slot_free_notify =
 			si->bdev->bd_disk->fops->swap_slot_free_notify;
@@ -752,6 +750,14 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 		offset++;
 	}
 	clear_shadow_from_swap_cache(si->type, begin, end);
+
+	/*
+	 * Make sure that try_to_unuse() observes si->inuse_pages reaching 0
+	 * only after the above cleanups are done.
+	 */
+	smp_wmb();
+	atomic_long_add(nr_entries, &nr_swap_pages);
+	WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
 }
 
 static void set_cluster_next(struct swap_info_struct *si, unsigned long next)
@@ -2049,7 +2055,7 @@ static int try_to_unuse(unsigned int type)
 	unsigned int i;
 
 	if (!READ_ONCE(si->inuse_pages))
-		return 0;
+		goto success;
 
 retry:
 	retval = shmem_unuse(type);
@@ -2130,6 +2136,12 @@ static int try_to_unuse(unsigned int type)
 		return -EINTR;
 	}
 
+success:
+	/*
+	 * Make sure that further cleanups after try_to_unuse() returns happen
+	 * after swap_range_free() reduces si->inuse_pages to 0.
+	 */
+	smp_mb();
 	return 0;
 }
 
-- 
2.43.0.429.g432eaa2c6b-goog