Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes: > 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> LGTM, Thanks! Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx> > --- > 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; > }
