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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 12 Aug 2009 20:07:43 +0530
Nitin Gupta <ngupta@vflare.org> 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 would be better if we could arrange for discard_swap() to be called
synchronously so we don't have to add a second parallel implementation
of a pretty similar thing.

Running the callback under spinlock is regrettable - it rather limits
what that handler can do.


> 
>  include/linux/swap.h |    5 +++++
>  mm/swapfile.c        |   16 ++++++++++++++++
>  2 files changed, 21 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 7c15334..4cbe3c4 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(unsigned, swap_free_notify_fn *);
>  struct backing_dev_info;
>  
>  /* linux/mm/thrash.c */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 8ffdc0d..aa95fc7 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -552,6 +552,20 @@ 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(unsigned type, swap_free_notify_fn *notify_fn)
> +{
> +	struct swap_info_struct *sis;
> +	sis = get_swap_info_struct(type);
> +	BUG_ON(!sis);
> +	sis->swap_free_notify_fn = notify_fn;
> +	return;
> +}
> +EXPORT_SYMBOL(set_swap_free_notify);

hm, well, spose so.  There's no provision here for multiple watchers,
the function is racy and makes no provision for telling the caller what
the old value was and the kernel will crash horridly if the module which
implements ->swap_free_notify_fn() gets rmmoded and forgets to do
set_swap_free_notify(..., NULL).  

But I guess we can live with those things.

>  static int swap_entry_free(struct swap_info_struct *p,
>  			   swp_entry_t ent, int cache)
>  {
> @@ -583,6 +597,8 @@ 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);