[PATCH] swap: get_swap_page drop swap_list_lock

Rewrite get_swap_page to allocate in just the same sequence as before, but
without holding swap_list_lock across its scan_swap_map.  Decrement
nr_swap_pages and update swap_list.next in advance, while still holding
swap_list_lock.  Skip full devices by testing highest_bit.  Swapoff hold
swap_device_lock as well as swap_list_lock to clear SWP_WRITEOK.  Reduces lock
contention when there are parallel swap devices of the same priority.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 36 insertions, 39 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 62e0da8f7e6e..e54d60af6b58 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -139,7 +139,6 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
                 }
                 si->swap_map[offset] = 1;
                 si->inuse_pages++;
-                nr_swap_pages--;
                 si->cluster_next = offset+1;
                 return offset;
         }
@@ -150,50 +149,45 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
 
 swp_entry_t get_swap_page(void)
 {
-        struct swap_info_struct * p;
-        unsigned long offset;
-        swp_entry_t entry;
-        int type, wrapped = 0;
+        struct swap_info_struct *si;
+        pgoff_t offset;
+        int type, next;
+        int wrapped = 0;
 
-        entry.val = 0;  /* Out of memory */
         swap_list_lock();
-        type = swap_list.next;
-        if (type < 0)
-                goto out;
         if (nr_swap_pages <= 0)
-                goto out;
-
-        while (1) {
-                p = &swap_info[type];
-                if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
-                        swap_device_lock(p);
-                        offset = scan_swap_map(p);
-                        swap_device_unlock(p);
-                        if (offset) {
-                                entry = swp_entry(type,offset);
-                                type = swap_info[type].next;
-                                if (type < 0 ||
-                                        p->prio != swap_info[type].prio) {
-                                                swap_list.next = swap_list.head;
-                                } else {
-                                        swap_list.next = type;
-                                }
-                                goto out;
-                        }
+                goto noswap;
+        nr_swap_pages--;
+
+        for (type = swap_list.next; type >= 0 && wrapped < 2; type = next) {
+                si = swap_info + type;
+                next = si->next;
+                if (next < 0 ||
+                    (!wrapped && si->prio != swap_info[next].prio)) {
+                        next = swap_list.head;
+                        wrapped++;
                 }
-                type = p->next;
-                if (!wrapped) {
-                        if (type < 0 || p->prio != swap_info[type].prio) {
-                                type = swap_list.head;
-                                wrapped = 1;
-                        }
-                } else
-                        if (type < 0)
-                                goto out;       /* out of swap space */
+
+                if (!si->highest_bit)
+                        continue;
+                if (!(si->flags & SWP_WRITEOK))
+                        continue;
+
+                swap_list.next = next;
+                swap_device_lock(si);
+                swap_list_unlock();
+                offset = scan_swap_map(si);
+                swap_device_unlock(si);
+                if (offset)
+                        return swp_entry(type, offset);
+                swap_list_lock();
+                next = swap_list.next;
         }
-out:
+
+        nr_swap_pages++;
+noswap:
         swap_list_unlock();
-        return entry;
+        return (swp_entry_t) {0};
 }
 
 static struct swap_info_struct * swap_info_get(swp_entry_t entry)
@@ -1105,8 +1099,11 @@ asmlinkage long sys_swapoff(const char __user * specialfile)
         }
         nr_swap_pages -= p->pages;
         total_swap_pages -= p->pages;
+        swap_device_lock(p);
         p->flags &= ~SWP_WRITEOK;
+        swap_device_unlock(p);
         swap_list_unlock();
+
         current->flags |= PF_SWAPOFF;
         err = try_to_unuse(type);
         current->flags &= ~PF_SWAPOFF;