Re: [PATCH 3/4] reuse unused swap entry if necessary

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Tue, 2 Jun 2009 12:12:02 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 
> This is a replacement for
>  mm-reuse-unused-swap-entry-if-necessary.patch in mmotm.
>  function is renamed and comments are added.
> 
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Now, we can know the swap is just used as SwapCache via swap_map,
> without looking up swap cache.
> 
> Then, we have a chance to reuse swap-cache-only swap entries in
> get_swap_pages().
> 
> This patch tries to free swap-cache-only swap entries if swap is
> not enough.
> Note: We hit following path when swap_cluster code cannot find
> a free cluster. Then, vm_swap_full() is not only condition to allow
> the kernel to reclaim unused swap.
> 
> Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

I've confirmed that usage increase of swap and swapcache stopped
at some threshold in my test, in which , before this patch, some programs
had been oom-killed after a long time because of shortage of swap space.

This has been merged to mm already though:

	Tested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

Thanks,
Daiuske Nishimura.

> ---
>  mm/swapfile.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
> 
> Index: mmotm-2.6.30-May28/mm/swapfile.c
> ===================================================================
> --- mmotm-2.6.30-May28.orig/mm/swapfile.c
> +++ mmotm-2.6.30-May28/mm/swapfile.c
> @@ -79,6 +79,32 @@ static inline unsigned short encode_swap
>  	return ret;
>  }
>  
> +/* returnes 1 if swap entry is freed */
> +static int
> +__try_to_reclaim_swap(struct swap_info_struct *si, unsigned long offset)
> +{
> +	int type = si - swap_info;
> +	swp_entry_t entry = swp_entry(type, offset);
> +	struct page *page;
> +	int ret = 0;
> +
> +	page = find_get_page(&swapper_space, entry.val);
> +	if (!page)
> +		return 0;
> +	/*
> +	 * This function is called from scan_swap_map() and it's called
> +	 * by vmscan.c at reclaiming pages. So, we hold a lock on a page, here.
> +	 * We have to use trylock for avoiding deadlock. This is a special
> +	 * case and you should use try_to_free_swap() with explicit lock_page()
> +	 * in usual operations.
> +	 */
> +	if (trylock_page(page)) {
> +		ret = try_to_free_swap(page);
> +		unlock_page(page);
> +	}
> +	page_cache_release(page);
> +	return ret;
> +}
>  
>  /*
>   * We need this because the bdev->unplug_fn can sleep and we cannot
> @@ -301,6 +327,19 @@ checks:
>  		goto no_page;
>  	if (offset > si->highest_bit)
>  		scan_base = offset = si->lowest_bit;
> +
> +	/* reuse swap entry of cache-only swap if not busy. */
> +	if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
> +		int swap_was_freed;
> +		spin_unlock(&swap_lock);
> +		swap_was_freed = __try_to_reclaim_swap(si, offset);
> +		spin_lock(&swap_lock);
> +		/* entry was freed successfully, try to use this again */
> +		if (swap_was_freed)
> +			goto checks;
> +		goto scan; /* check next one */
> +	}
> +
>  	if (si->swap_map[offset])
>  		goto scan;
>  
> @@ -382,6 +421,10 @@ scan:
>  			spin_lock(&swap_lock);
>  			goto checks;
>  		}
> +		if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
> +			spin_lock(&swap_lock);
> +			goto checks;
> +		}
>  		if (unlikely(--latency_ration < 0)) {
>  			cond_resched();
>  			latency_ration = LATENCY_LIMIT;
> @@ -393,6 +436,10 @@ scan:
>  			spin_lock(&swap_lock);
>  			goto checks;
>  		}
> +		if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
> +			spin_lock(&swap_lock);
> +			goto checks;
> +		}
>  		if (unlikely(--latency_ration < 0)) {
>  			cond_resched();
>  			latency_ration = LATENCY_LIMIT;
> 

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