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Date:	Mon, 28 Dec 2009 08:13:06 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] [mmotm] Add notifiers for various swap events

On Mon, Dec 28, 2009 at 3:19 AM, Nitin Gupta <ngupta@...are.org> wrote:
> Events:
>  - Swapon
>  - Swapoff
>  - When a swap slot is freed
>
> This is required for ramzswap module which implements RAM based block
> devices to be used as swap disks. These devices require a notification
> on these events to function properly.
>
> Currently, I'm not sure if any of these event notifiers have any other
> users. However, adding ramzswap specific hooks instead of this generic
> approach resulted in a bad/hacky code.
>
> For SWAP_EVENT_SLOT_FREE, callbacks are made under swap_lock. Currently, this
> is not a problem since ramzswap is the only user and the callback it registers
> can be safely made under this lock. However, if this event finds more users,
> we might have to work on reducing contention on this lock (per-swap lock?).
>
> Signed-off-by: Nitin Gupta <ngupta@...are.org>

This looks good to me. Hugh? Nitin, does this patch cause any
performance hit for the !CONFIG_RAMZSWAP case?

> ---
>  include/linux/swap.h |   12 +++++++++
>  mm/swapfile.c        |   67 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 79 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index a2602a8..5fd2ac7 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -150,6 +150,12 @@ enum {
>        SWP_SCANNING    = (1 << 8),     /* refcount in scan_swap_map */
>  };
>
> +enum swap_event {
> +       SWAP_EVENT_SWAPON,
> +       SWAP_EVENT_SWAPOFF,
> +       SWAP_EVENT_SLOT_FREE,
> +};
> +
>  #define SWAP_CLUSTER_MAX 32
>
>  #define SWAP_MAP_MAX   0x3e    /* Max duplication count, in first swap_map */
> @@ -180,6 +186,7 @@ struct swap_info_struct {
>        struct swap_extent *curr_swap_extent;
>        struct swap_extent first_swap_extent;
>        struct block_device *bdev;      /* swap device or bdev of swap file */
> +       struct atomic_notifier_head slot_free_notify_list;
>        struct file *swap_file;         /* seldom referenced */
>        unsigned int old_block_size;    /* seldom referenced */
>  };
> @@ -329,8 +336,13 @@ extern sector_t map_swap_page(struct page *, struct block_device **);
>  extern sector_t swapdev_block(int, pgoff_t);
>  extern int reuse_swap_page(struct page *);
>  extern int try_to_free_swap(struct page *);
> +extern int register_swap_event_notifier(struct notifier_block *nb,
> +                       enum swap_event event, unsigned long val);
> +extern int unregister_swap_event_notifier(struct notifier_block *nb,
> +                       enum swap_event event, unsigned long val);
>  struct backing_dev_info;
>
> +
>  /* linux/mm/thrash.c */
>  extern struct mm_struct *swap_token_mm;
>  extern void grab_swap_token(struct mm_struct *);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 6c0585b..301905d 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -57,6 +57,9 @@ static struct swap_list_t swap_list = {-1, -1};
>  static struct swap_info_struct *swap_info[MAX_SWAPFILES];
>
>  static DEFINE_MUTEX(swapon_mutex);
> +static BLOCKING_NOTIFIER_HEAD(swapon_notify_list);
> +static BLOCKING_NOTIFIER_HEAD(swapoff_notify_list);
> +
>
>  static inline unsigned char swap_count(unsigned char ent)
>  {
> @@ -583,6 +586,8 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
>                        swap_list.next = p->type;
>                nr_swap_pages++;
>                p->inuse_pages--;
> +               atomic_notifier_call_chain(&p->slot_free_notify_list,
> +                                       offset, p->swap_file);
>        }
>
>        return usage;
> @@ -1609,6 +1614,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>        p->swap_map = NULL;
>        p->flags = 0;
>        spin_unlock(&swap_lock);
> +       blocking_notifier_call_chain(&swapoff_notify_list, type, swap_file);
>        mutex_unlock(&swapon_mutex);
>        vfree(swap_map);
>        /* Destroy swap account informatin */
> @@ -2022,7 +2028,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>                swap_list.head = swap_list.next = type;
>        else
>                swap_info[prev]->next = type;
> +       ATOMIC_INIT_NOTIFIER_HEAD(&p->slot_free_notify_list);
>        spin_unlock(&swap_lock);
> +       blocking_notifier_call_chain(&swapon_notify_list, type, swap_file);
>        mutex_unlock(&swapon_mutex);
>        error = 0;
>        goto out;
> @@ -2446,3 +2454,62 @@ static void free_swap_count_continuations(struct swap_info_struct *si)
>                }
>        }
>  }
> +
> +int register_swap_event_notifier(struct notifier_block *nb,
> +                               enum swap_event event, unsigned long val)
> +{
> +       switch (event) {
> +       case SWAP_EVENT_SWAPON:
> +               return blocking_notifier_chain_register(
> +                                       &swapon_notify_list, nb);
> +       case SWAP_EVENT_SWAPOFF:
> +               return blocking_notifier_chain_register(
> +                                       &swapoff_notify_list, nb);
> +       case SWAP_EVENT_SLOT_FREE:
> +               {
> +               struct swap_info_struct *sis;
> +
> +               if (val > nr_swapfiles)
> +                       goto out;
> +               sis = swap_info[val];
> +               return atomic_notifier_chain_register(
> +                               &sis->slot_free_notify_list, nb);
> +               }
> +       default:
> +               pr_err("Invalid swap event: %d\n", event);
> +       };
> +
> +out:
> +       return -EINVAL;
> +}
> +EXPORT_SYMBOL_GPL(register_swap_event_notifier);
> +
> +int unregister_swap_event_notifier(struct notifier_block *nb,
> +                               enum swap_event event, unsigned long val)
> +{
> +       switch (event) {
> +       case SWAP_EVENT_SWAPON:
> +               return blocking_notifier_chain_unregister(
> +                                       &swapon_notify_list, nb);
> +       case SWAP_EVENT_SWAPOFF:
> +               return blocking_notifier_chain_unregister(
> +                                       &swapoff_notify_list, nb);
> +       case SWAP_EVENT_SLOT_FREE:
> +               {
> +               struct swap_info_struct *sis;
> +
> +               if (val > nr_swapfiles)
> +                       goto out;
> +               sis = swap_info[val];
> +               return atomic_notifier_chain_unregister(
> +                               &sis->slot_free_notify_list, nb);
> +               }
> +       default:
> +               pr_err("Invalid swap event: %d\n", event);
> +       };
> +
> +out:
> +       return -EINVAL;
> +}
> +EXPORT_SYMBOL_GPL(unregister_swap_event_notifier);
> +
> --
> 1.6.2.5
>
> --
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