[RFC][PATCH 3/3] count swap caches whose swp_entry can be freed.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

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

Count a swap entry which is just a swapcache.
i.e. swap entry can be freed immediately.

This counter tells us there is a chance to reclaim swap entries.
Maybe good for mem+swap controller.
(Freeing routine itself is a homework...)

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 include/linux/swap.h |    1 +
 mm/swapfile.c        |   15 ++++++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

Index: mmotm-2.6.30-May17/include/linux/swap.h
===================================================================
--- mmotm-2.6.30-May17.orig/include/linux/swap.h
+++ mmotm-2.6.30-May17/include/linux/swap.h
@@ -156,6 +156,7 @@ struct swap_info_struct {
 	unsigned int max;
 	unsigned int inuse_pages;
 	unsigned int old_block_size;
+	unsigned int orphan_swap_cache;
 };
 
 struct swap_list_t {
Index: mmotm-2.6.30-May17/mm/swapfile.c
===================================================================
--- mmotm-2.6.30-May17.orig/mm/swapfile.c
+++ mmotm-2.6.30-May17/mm/swapfile.c
@@ -291,9 +291,10 @@ checks:
 		si->lowest_bit = si->max;
 		si->highest_bit = 0;
 	}
-	if (cache)
+	if (cache) {
 		si->swap_map[offset] = SWAP_HAS_CACHE; /* via get_swap_page() */
-	else
+		si->orphan_swap_cache++;
+	} else
 		si->swap_map[offset] = 1; /* via alloc_swap_block()  */
 
 	si->cluster_next = offset + 1;
@@ -521,9 +522,14 @@ static int swap_entry_free(struct swap_i
 			swap_list.next = p - swap_info;
 		nr_swap_pages++;
 		p->inuse_pages--;
+		if (cache)
+			p->orphan_swap_cache--;
 	}
-	if (!swap_has_ref(count))
+	if (!swap_has_ref(count)) {
 		mem_cgroup_uncharge_swap(ent);
+		if (count & SWAP_HAS_CACHE)
+			p->orphan_swap_cache++;
+	}
 	return count;
 }
 
@@ -2022,6 +2028,9 @@ int swap_duplicate(swp_entry_t entry)
 		goto out_unlock;
 
 	count = p->swap_map[offset] & SWAP_MAP_MASK;
+	if (!count && (p->swap_map[offset] & SWAP_HAS_CACHE))
+		p->orphan_swap_cache++;
+
 	if (count < SWAP_MAP_MAX - 1) {
 		p->swap_map[offset] += 1;
 		result = 1;

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