Re: [PATCH 8/9] mm/z3fold: fix z3fold_reclaim_page races with z3fold_free

[Vitaly Wool <vitaly.wool@konsulko.com>]

On Fri, Apr 29, 2022 at 8:40 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> Think about the below scene:
>
> CPU1                            CPU2
> z3fold_reclaim_page             z3fold_free
>  spin_lock(&pool->lock)          get_z3fold_header -- hold page_lock
>  kref_get_unless_zero
>                                  kref_put--zhdr->refcount can be 1 now
>  !z3fold_page_trylock
>   kref_put -- zhdr->refcount is 0 now
>    release_z3fold_page
>     WARN_ON(!list_empty(&zhdr->buddy)); -- we're on buddy now!
>     spin_lock(&pool->lock); -- deadlock here!
>
> z3fold_reclaim_page might race with z3fold_free and will lead to pool lock
> deadlock and zhdr buddy non-empty warning. To fix this, defer getting the
> refcount until page_lock is held just like what __z3fold_alloc does. Note
> this has the side effect that we won't break the reclaim if we meet a soon
> to be released z3fold page now.
>
> Fixes: dcf5aedb24f8 ("z3fold: stricter locking and more careful reclaim")
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/z3fold.c | 18 +++---------------
>  1 file changed, 3 insertions(+), 15 deletions(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 4a3cd2ff15b0..a7769befd74e 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -519,13 +519,6 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
>         atomic64_dec(&pool->pages_nr);
>  }
>
> -static void release_z3fold_page(struct kref *ref)
> -{
> -       struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
> -                                               refcount);
> -       __release_z3fold_page(zhdr, false);
> -}
> -
>  static void release_z3fold_page_locked(struct kref *ref)
>  {
>         struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
> @@ -1317,12 +1310,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>                                 break;
>                         }
>
> -                       if (kref_get_unless_zero(&zhdr->refcount) == 0) {
> -                               zhdr = NULL;
> -                               break;
> -                       }
>                         if (!z3fold_page_trylock(zhdr)) {
> -                               kref_put(&zhdr->refcount, release_z3fold_page);
>                                 zhdr = NULL;
>                                 continue; /* can't evict at this point */
>                         }
> @@ -1333,14 +1321,14 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>                          */
>                         if (zhdr->foreign_handles ||
>                             test_and_set_bit(PAGE_CLAIMED, &page->private)) {
> -                               if (!kref_put(&zhdr->refcount,
> -                                               release_z3fold_page_locked))
> -                                       z3fold_page_unlock(zhdr);
> +                               z3fold_page_unlock(zhdr);
>                                 zhdr = NULL;
>                                 continue; /* can't evict such page */
>                         }
>                         list_del_init(&zhdr->buddy);
>                         zhdr->cpu = -1;
> +                       /* See comment in __z3fold_alloc. */
> +                       kref_get(&zhdr->refcount);
>                         break;
>                 }
>

Reviewed-by: Vitaly Wool <vitaly.wool@konsulko.com>
> --
> 2.23.0
>