Re: [PATCH 2/4] send callback when swap slot is freed

[Pekka Enberg <penberg@cs.helsinki.fi>]

On Fri, 2009-09-18 at 04:13 +0530, Nitin Gupta wrote:
> Currently, we have "swap discard" mechanism which sends a discard bio request
> when we find a free cluster during scan_swap_map(). This callback can come a
> long time after swap slots are actually freed.
> 
> This delay in callback is a great problem when (compressed) RAM [1] is used
> as a swap device. So, this change adds a callback which is called as
> soon as a swap slot becomes free. For above mentioned case of swapping
> over compressed RAM device, this is very useful since we can immediately
> free memory allocated for this swap page.
> 
> This callback does not replace swap discard support. It is called with
> swap_lock held, so it is meant to trigger action that finishes quickly.
> However, swap discard is an I/O request and can be used for taking longer
> actions.
> 
> It is preferred to use this callback for ramzswap case even if discard
> mechanism could be improved such that it can be called as often as required.
> This is because, allocation of 'bio'(s) is undesirable since ramzswap always
> operates under low memory conditions (its a swap device). Also, batching of
> discard bio requests is not optimal since stale data can accumulate very
> quickly in ramzswap devices, pushing system further into low memory state.
> 
> Links:
> [1] http://compcache.googlecode.com/
> 
> Signed-off-by: Nitin Gupta <ngupta@vflare.org>
> 
> ---
>  include/linux/swap.h |    5 +++++
>  mm/swapfile.c        |   34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 7c15334..64796fc 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -8,6 +8,7 @@
>  #include <linux/memcontrol.h>
>  #include <linux/sched.h>
>  #include <linux/node.h>
> +#include <linux/blkdev.h>
> 
>  #include <asm/atomic.h>
>  #include <asm/page.h>
> @@ -20,6 +21,8 @@ struct bio;
>  #define SWAP_FLAG_PRIO_MASK	0x7fff
>  #define SWAP_FLAG_PRIO_SHIFT	0
> 
> +typedef void (swap_free_notify_fn) (struct block_device *, unsigned long);
> +
>  static inline int current_is_kswapd(void)
>  {
>  	return current->flags & PF_KSWAPD;
> @@ -155,6 +158,7 @@ struct swap_info_struct {
>  	unsigned int max;
>  	unsigned int inuse_pages;
>  	unsigned int old_block_size;
> +	swap_free_notify_fn *swap_free_notify_fn;
>  };
> 
>  struct swap_list_t {
> @@ -295,6 +299,7 @@ extern sector_t swapdev_block(int, pgoff_t);
>  extern struct swap_info_struct *get_swap_info_struct(unsigned);
>  extern int reuse_swap_page(struct page *);
>  extern int try_to_free_swap(struct page *);
> +extern void set_swap_free_notify(struct block_device *, swap_free_notify_fn *);
>  struct backing_dev_info;
> 
>  /* linux/mm/thrash.c */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 74f1102..b165db0 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -554,6 +554,38 @@ out:
>  	return NULL;
>  }
> 
> +/*
> + * Sets callback for event when swap_map[offset] == 0
> + * i.e. page at this swap offset is no longer used.
> + */
> +void set_swap_free_notify(struct block_device *bdev,
> +			swap_free_notify_fn *notify_fn)
> +{
> +	unsigned int i;
> +	struct swap_info_struct *sis;
> +
> +	spin_lock(&swap_lock);
> +	for (i = 0; i <= nr_swapfiles; i++) {
> +		sis = &swap_info[i];
> +		if (!(sis->flags & SWP_USED))
> +			continue;
> +		if (sis->bdev == bdev)
> +			break;
> +	}
> +
> +	/* swap device not found */
> +	if (i > nr_swapfiles) {
> +		spin_unlock(&swap_lock);
> +		return;
> +	}
> +
> +	BUG_ON(!sis || sis->swap_free_notify_fn);
> +	sis->swap_free_notify_fn = notify_fn;
> +	spin_unlock(&swap_lock);
> +	return;
> +}
> +EXPORT_SYMBOL_GPL(set_swap_free_notify);
> +
>  static int swap_entry_free(struct swap_info_struct *p,
>  			   swp_entry_t ent, int cache)
>  {
> @@ -585,6 +617,8 @@ static int swap_entry_free(struct swap_info_struct *p,
>  			swap_list.next = p - swap_info;
>  		nr_swap_pages++;
>  		p->inuse_pages--;
> +		if (p->swap_free_notify_fn)
> +			p->swap_free_notify_fn(p->bdev, offset);
>  	}
>  	if (!swap_count(count))
>  		mem_cgroup_uncharge_swap(ent);

OK, this hits core kernel code so we need to CC some more mm/swapfile.c
people. The set_swap_free_notify() API looks strange to me. Hugh, I
think you mentioned that you're okay with an explicit hook. Any
suggestions how to do this cleanly?

			Pekka

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