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

KAMEZAWA Hiroyuki
2009-05-28T05:22:31Z

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Now, 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>
---
mm/swapfile.c | 39 +++++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)

Index: new-trial-swapcount2/mm/swapfile.c
===================================================================
--- new-trial-swapcount2.orig/mm/swapfile.c
+++ new-trial-swapcount2/mm/swapfile.c
@@ -73,6 +73,25 @@ static inline unsigned short make_swap_c
return ret;
}

+static int
+try_to_reuse_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;
+ 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
* hold swap_lock while calling the unplug_fn. And swap_lock
@@ -294,6 +313,18 @@ 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 ret;
+ spin_unlock(&swap_lock);
+ ret = try_to_reuse_swap(si, offset);
+ spin_lock(&swap_lock);
+ if (ret)
+ goto checks; /* we released swap_lock. retry. */
+ goto scan; /* In some racy case */
+ }
+
if (si->swap_map[offset])
goto scan;

@@ -375,6 +406,10 @@ 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;
@@ -386,6 +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;