mm: swap: enforce updating inuse_pages at the end of swap_range_free()

Patch series "mm: zswap: simplify zswap_swapoff()", v2.

These patches aim to simplify zswap_swapoff() by removing the unnecessary
trees cleanup code.  Patch 1 makes sure that the order of operations
during swapoff is enforced correctly, making sure the simplification in
patch 2 is correct in a future-proof manner.


This patch (of 2):

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().

Link: https://lkml.kernel.org/r/20240124045113.415378-1-yosryahmed@google.com
Link: https://lkml.kernel.org/r/20240124045113.415378-2-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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;
 }