linux-kernel - Re: [PATCH] mm: fix a race on nr_swap

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Date:   Thu, 3 Dec 2020 20:19:51 +0800
From:   Zhaoyang Huang <huangzhaoyang@...il.com>
To:     Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix a race on nr_swap_pages

It is show_swap_cache_info() which races with get_swap_xxx

On Thu, Dec 3, 2020 at 7:36 PM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
>
> The scenario on which "Free swap -4kB" happens in my system, which is caused by
>  get_swap_page_of_type or get_swap_pages racing with show_mem. Remove the race
>  here.
>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> ---
>  mm/swapfile.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index cf63b5f..13201b6 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -974,6 +974,8 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
>         /* Only single cluster request supported */
>         WARN_ON_ONCE(n_goal > 1 && size == SWAPFILE_CLUSTER);
>
> +       spin_lock(&swap_avail_lock);
> +
>         avail_pgs = atomic_long_read(&nr_swap_pages) / size;
>         if (avail_pgs <= 0)
>                 goto noswap;
> @@ -986,8 +988,6 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
>
>         atomic_long_sub(n_goal * size, &nr_swap_pages);
>
> -       spin_lock(&swap_avail_lock);
> -
>  start_over:
>         node = numa_node_id();
>         plist_for_each_entry_safe(si, next, &swap_avail_heads[node], avail_lists[node]) {
> @@ -1061,14 +1061,13 @@ swp_entry_t get_swap_page_of_type(int type)
>
>         spin_lock(&si->lock);
>         if (si->flags & SWP_WRITEOK) {
> -               atomic_long_dec(&nr_swap_pages);
>                 /* This is called for allocating swap entry, not cache */
>                 offset = scan_swap_map(si, 1);
>                 if (offset) {
> +                       atomic_long_dec(&nr_swap_pages);
>                         spin_unlock(&si->lock);
>                         return swp_entry(type, offset);
>                 }
> -               atomic_long_inc(&nr_swap_pages);
>         }
>         spin_unlock(&si->lock);
>  fail:
> --
> 1.9.1
>