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.com
Signed-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>

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 0c6dde8b8604d89d6e9543c393442db8ea999d34..a8edaf4e5b8ad94f4f594ba7d1dcd201072e44af 100644 (file)
--- 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 @@ retry:
                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;
 }