[PATCH 1/2] mm: swap: update inuse_pages after all cleanups are done

[Yosry Ahmed]

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() uses inuse_pages to make sure all swap entries
are freed. Make sure we only update inuse_pages after we are done with
the cleanups.

In practice, this shouldn't matter, because swap_range_free() is called
with the swap info lock held, and the swapoff code will spin for that
lock after try_to_unuse() anyway.

The goal is to make it obvious and more future proof that once
try_to_unuse() returns, all cleanups are done. This also facilitates a
following zswap cleanup patch which uses this fact to simplify
zswap_swapoff().

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 mm/swapfile.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 556ff7347d5f0..2fedb148b9404 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 +750,8 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 		offset++;
 	}
 	clear_shadow_from_swap_cache(si->type, begin, end);
+	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)
-- 
2.43.0.429.g432eaa2c6b-goog