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

["Huang, Ying" <ying.huang@intel.com>]

Yosry Ahmed <yosryahmed@google.com> 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() 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.

Defines "all cleanups".  Apparently, some other operations are still
to be done after try_to_unuse() in swap_off().

> This also facilitates a
> following zswap cleanup patch which uses this fact to simplify
> zswap_swapoff().
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  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);

This isn't enough.  You need to use smp_wmb() here and smp_rmb() in
somewhere reading si->inuse_pages.

>  }
>  
>  static void set_cluster_next(struct swap_info_struct *si, unsigned long next)

--
Best Regards,
Huang, Ying