Re: [PATCH v1 2/5] mm: add full variable in swap_info_struct

[Dan Streetman <ddstreet@ieee.org>]

On Sun, Sep 21, 2014 at 8:03 PM, Minchan Kim <minchan@kernel.org> wrote:
> Now, swap leans on !p->highest_bit to indicate a swap is full.
> It works well for normal swap because every slot on swap device
> is used up when the swap is full but in case of zram, swap sees
> still many empty slot although backed device(ie, zram) is full
> since zram's limit is over so that it could make trouble when
> swap use highest_bit to select new slot via free_cluster.
>
> This patch introduces full varaiable in swap_info_struct
> to solve the problem.
>
> Suggested-by: Dan Streetman <ddstreet@ieee.org>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  include/linux/swap.h |  1 +
>  mm/swapfile.c        | 33 +++++++++++++++++++--------------
>  2 files changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index ea4f926e6b9b..a3c11c051495 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -224,6 +224,7 @@ struct swap_info_struct {
>         struct swap_cluster_info free_cluster_tail; /* free cluster list tail */
>         unsigned int lowest_bit;        /* index of first free in swap_map */
>         unsigned int highest_bit;       /* index of last free in swap_map */
> +       bool    full;                   /* whether swap is full or not */
>         unsigned int pages;             /* total of usable pages of swap */
>         unsigned int inuse_pages;       /* number of those currently in use */
>         unsigned int cluster_next;      /* likely index for next allocation */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index c07f7f4912e9..209112cf8b83 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -558,7 +558,7 @@ checks:
>         }
>         if (!(si->flags & SWP_WRITEOK))
>                 goto no_page;
> -       if (!si->highest_bit)
> +       if (si->full)
>                 goto no_page;
>         if (offset > si->highest_bit)
>                 scan_base = offset = si->lowest_bit;
> @@ -589,6 +589,7 @@ checks:
>                 spin_lock(&swap_avail_lock);
>                 plist_del(&si->avail_list, &swap_avail_head);
>                 spin_unlock(&swap_avail_lock);
> +               si->full = true;
>         }
>         si->swap_map[offset] = usage;
>         inc_cluster_info_page(si, si->cluster_info, offset);
> @@ -653,14 +654,14 @@ start_over:
>                 plist_requeue(&si->avail_list, &swap_avail_head);
>                 spin_unlock(&swap_avail_lock);
>                 spin_lock(&si->lock);
> -               if (!si->highest_bit || !(si->flags & SWP_WRITEOK)) {
> +               if (si->full || !(si->flags & SWP_WRITEOK)) {
>                         spin_lock(&swap_avail_lock);
>                         if (plist_node_empty(&si->avail_list)) {
>                                 spin_unlock(&si->lock);
>                                 goto nextsi;
>                         }
> -                       WARN(!si->highest_bit,
> -                            "swap_info %d in list but !highest_bit\n",
> +                       WARN(si->full,
> +                            "swap_info %d in list but swap is full\n",
>                              si->type);
>                         WARN(!(si->flags & SWP_WRITEOK),
>                              "swap_info %d in list but !SWP_WRITEOK\n",
> @@ -796,21 +797,25 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
>
>         /* free if no reference */
>         if (!usage) {
> +               bool was_full;
> +
>                 dec_cluster_info_page(p, p->cluster_info, offset);
>                 if (offset < p->lowest_bit)
>                         p->lowest_bit = offset;
> -               if (offset > p->highest_bit) {
> -                       bool was_full = !p->highest_bit;
> +               if (offset > p->highest_bit)
>                         p->highest_bit = offset;
> -                       if (was_full && (p->flags & SWP_WRITEOK)) {
> -                               spin_lock(&swap_avail_lock);
> -                               WARN_ON(!plist_node_empty(&p->avail_list));
> -                               if (plist_node_empty(&p->avail_list))
> -                                       plist_add(&p->avail_list,
> -                                                 &swap_avail_head);
> -                               spin_unlock(&swap_avail_lock);
> -                       }
> +               was_full = p->full;
> +
> +               if (was_full && (p->flags & SWP_WRITEOK)) {

was_full was only needed because highest_bit was reset to offset right
before checking for fullness, so now that ->full is used instead of
!highest_bit, was_full isn't needed anymore, you can just check
p->full.


> +                       spin_lock(&swap_avail_lock);
> +                       WARN_ON(!plist_node_empty(&p->avail_list));
> +                       if (plist_node_empty(&p->avail_list))
> +                               plist_add(&p->avail_list,
> +                                         &swap_avail_head);
> +                       spin_unlock(&swap_avail_lock);
> +                       p->full = false;
>                 }
> +
>                 atomic_long_inc(&nr_swap_pages);
>                 p->inuse_pages--;
>                 frontswap_invalidate_page(p->type, offset);
> --
> 2.0.0
>

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