mm: reuse unused swap entry if necessary

Presently we can know a swap entry 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.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Balbir Singh <balbir@in.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Tested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 0d72969..28faa01 100644 (file)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -79,6 +79,32 @@ static inline unsigned short encode_swapmap(int count, bool has_cache)
        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;