On 12/4/20 3:52 AM, Zhaoyang Huang wrote:
> The scenario on which "Free swap = -4kB" happens in my system, which is caused
> by several get_swap_pages racing with each other and show_swap_cache_info
> happens simutaniously. No need to add a lock on get_swap_page_of_type as we
> remove "Presub/PosAdd" here.
>
> ProcessA ProcessB ProcessC
> ngoals = 1 ngoals = 1
> avail = nr_swap_pages(1) avail = nr_swap_pages(1)
> nr_swap_pages(1) -= ngoals
> nr_swap_pages(0) -= ngoals
> nr_swap_pages = -1
>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
Better now.
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> change of v2: fix bug of unpaired of spin_lock
> ---
> ---
> mm/swapfile.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index cf63b5f..1212f17 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -974,9 +974,13 @@ 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)
> + if (avail_pgs <= 0) {
> + spin_unlock(&swap_avail_lock);
> goto noswap;
> + }
>
> if (n_goal > SWAP_BATCH)
> n_goal = SWAP_BATCH;
> @@ -986,8 +990,6 @@ 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 +1063,13 @@ 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:
>
