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@xxxxxxxxxx> --- 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
