linux-kernel - Re: [PATCH 7/9] mm/z3fold: always clear PAGE

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Date:   Thu, 19 May 2022 09:14:18 +0200
From:   Vitaly Wool <vitaly.wool@...sulko.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/9] mm/z3fold: always clear PAGE_CLAIMED under z3fold
 page lock

On Fri, Apr 29, 2022 at 8:40 AM Miaohe Lin <linmiaohe@...wei.com> wrote:
>
> Think about the below race window:
>
> CPU1                            CPU2
> z3fold_reclaim_page             z3fold_free
>  test_and_set_bit PAGE_CLAIMED
>  failed to reclaim page
>  z3fold_page_lock(zhdr);
>  add back to the lru list;
>  z3fold_page_unlock(zhdr);
>                                  get_z3fold_header
>                                  page_claimed=test_and_set_bit PAGE_CLAIMED
>
>  clear_bit(PAGE_CLAIMED, &page->private);
>
>                                  if (!page_claimed) /* it's false true */
>                                   free_handle is not called
>
> free_handle won't be called in this case. So z3fold_buddy_slots will leak.
> Fix it by always clear PAGE_CLAIMED under z3fold page lock.
>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/z3fold.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index a1c150fc8def..4a3cd2ff15b0 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -1221,8 +1221,8 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
>                 return;
>         }
>         if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
> -               put_z3fold_header(zhdr);
>                 clear_bit(PAGE_CLAIMED, &page->private);
> +               put_z3fold_header(zhdr);
>                 return;
>         }
>         if (zhdr->cpu < 0 || !cpu_online(zhdr->cpu)) {
> @@ -1424,8 +1424,8 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>                         spin_unlock(&pool->lock);
>                         if (list_empty(&zhdr->buddy))
>                                 add_to_unbuddied(pool, zhdr);
> -                       z3fold_page_unlock(zhdr);
>                         clear_bit(PAGE_CLAIMED, &page->private);
> +                       z3fold_page_unlock(zhdr);
>                 }
>
>                 /* We started off locked to we need to lock the pool back */
> @@ -1577,8 +1577,8 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
>         if (!z3fold_page_trylock(zhdr))
>                 return -EAGAIN;
>         if (zhdr->mapped_count != 0 || zhdr->foreign_handles != 0) {
> -               z3fold_page_unlock(zhdr);
>                 clear_bit(PAGE_CLAIMED, &page->private);
> +               z3fold_page_unlock(zhdr);
>                 return -EBUSY;
>         }
>         if (work_pending(&zhdr->work)) {

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