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

Daisuke Nishimura
2009-06-04T06:23:39Z

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;
>