[PATCH] swap: swap_lock replace list+device

The idea of a swap_device_lock per device, and a swap_list_lock over them all,
is appealing; but in practice almost every holder of swap_device_lock must
already hold swap_list_lock, which defeats the purpose of the split.

The only exceptions have been swap_duplicate, valid_swaphandles and an
untrodden path in try_to_unuse (plus a few places added in this series).
valid_swaphandles doesn't show up high in profiles, but swap_duplicate does
demand attention.  However, with the hold time in get_swap_pages so much
reduced, I've not yet found a load and set of swap device priorities to show
even swap_duplicate benefitting from the split.  Certainly the split is mere
overhead in the common case of a single swap device.

So, replace swap_list_lock and swap_device_lock by spinlock_t swap_lock
(generally we seem to prefer an _ in the name, and not hide in a macro).

If someone can show a regression in swap_duplicate, then probably we should
add a hashlock for the swap_map entries alone (shorts being anatomic), so as
to help the case of the single swap device too.

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

5 files changed, 66 insertions, 95 deletions

diff --git a/Documentation/vm/locking b/Documentation/vm/locking
index c3ef09ae3bb1..f366fa956179 100644
--- a/Documentation/vm/locking
+++ b/Documentation/vm/locking
@@ -83,19 +83,18 @@ single address space optimization, so that the zap_page_range (from
 vmtruncate) does not lose sending ipi's to cloned threads that might 
 be spawned underneath it and go to user mode to drag in pte's into tlbs.
 
-swap_list_lock/swap_device_lock
--------------------------------
+swap_lock
+--------------
 The swap devices are chained in priority order from the "swap_list" header. 
 The "swap_list" is used for the round-robin swaphandle allocation strategy.
 The #free swaphandles is maintained in "nr_swap_pages". These two together
-are protected by the swap_list_lock. 
+are protected by the swap_lock.
 
-The swap_device_lock, which is per swap device, protects the reference 
-counts on the corresponding swaphandles, maintained in the "swap_map"
-array, and the "highest_bit" and "lowest_bit" fields.
+The swap_lock also protects all the device reference counts on the
+corresponding swaphandles, maintained in the "swap_map" array, and the
+"highest_bit" and "lowest_bit" fields.
 
-Both of these are spinlocks, and are never acquired from intr level. The
-locking hierarchy is swap_list_lock -> swap_device_lock.
+The swap_lock is a spinlock, and is never acquired from intr level.
 
 To prevent races between swap space deletion or async readahead swapins
 deciding whether a swap handle is being used, ie worthy of being read in
diff --git a/include/linux/swap.h b/include/linux/swap.h
index db3b5de7c92f..3c9ff0048153 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -121,7 +121,7 @@ enum {
  */
 struct swap_info_struct {
         unsigned int flags;
-        spinlock_t sdev_lock;
+        int prio;                       /* swap priority */
         struct file *swap_file;
         struct block_device *bdev;
         struct list_head extent_list;
@@ -135,7 +135,6 @@ struct swap_info_struct {
         unsigned int pages;
         unsigned int max;
         unsigned int inuse_pages;
-        int prio;                       /* swap priority */
         int next;                       /* next entry on swap list */
 };
 
@@ -221,13 +220,7 @@ extern int can_share_swap_page(struct page *);
 extern int remove_exclusive_swap_page(struct page *);
 struct backing_dev_info;
 
-extern struct swap_list_t swap_list;
-extern spinlock_t swaplock;
-
-#define swap_list_lock()        spin_lock(&swaplock)
-#define swap_list_unlock()      spin_unlock(&swaplock)
-#define swap_device_lock(p)     spin_lock(&p->sdev_lock)
-#define swap_device_unlock(p)   spin_unlock(&p->sdev_lock)
+extern spinlock_t swap_lock;
 
 /* linux/mm/thrash.c */
 extern struct mm_struct * swap_token_mm;
diff --git a/mm/filemap.c b/mm/filemap.c
index c11418dd94e8..edc54436fa94 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -54,9 +54,8 @@
  *
  *  ->i_mmap_lock               (vmtruncate)
  *    ->private_lock            (__free_pte->__set_page_dirty_buffers)
- *      ->swap_list_lock
- *        ->swap_device_lock    (exclusive_swap_page, others)
- *          ->mapping->tree_lock
+ *      ->swap_lock             (exclusive_swap_page, others)
+ *        ->mapping->tree_lock
  *
  *  ->i_sem
  *    ->i_mmap_lock             (truncate->unmap_mapping_range)
@@ -86,7 +85,7 @@
  *    ->page_table_lock         (anon_vma_prepare and various)
  *
  *  ->page_table_lock
- *    ->swap_device_lock        (try_to_unmap_one)
+ *    ->swap_lock               (try_to_unmap_one)
  *    ->private_lock            (try_to_unmap_one)
  *    ->tree_lock               (try_to_unmap_one)
  *    ->zone.lru_lock           (follow_page->mark_page_accessed)
diff --git a/mm/rmap.c b/mm/rmap.c
index 08ac5c7fa91f..facb8cdca665 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -34,9 +34,8 @@
  *       anon_vma->lock
  *         mm->page_table_lock
  *           zone->lru_lock (in mark_page_accessed)
- *           swap_list_lock (in swap_free etc's swap_info_get)
+ *           swap_lock (in swap_duplicate, swap_info_get)
  *             mmlist_lock (in mmput, drain_mmlist and others)
- *             swap_device_lock (in swap_duplicate, swap_info_get)
  *             mapping->private_lock (in __set_page_dirty_buffers)
  *             inode_lock (in set_page_dirty's __mark_inode_dirty)
  *               sb_lock (within inode_lock in fs/fs-writeback.c)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index e675ae55f87d..4b6e8bf986bc 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -31,7 +31,7 @@
 #include <asm/tlbflush.h>
 #include <linux/swapops.h>
 
-DEFINE_SPINLOCK(swaplock);
+DEFINE_SPINLOCK(swap_lock);
 unsigned int nr_swapfiles;
 long total_swap_pages;
 static int swap_overflow;
@@ -51,7 +51,7 @@ static DECLARE_MUTEX(swapon_sem);
 
 /*
  * We need this because the bdev->unplug_fn can sleep and we cannot
- * hold swap_list_lock while calling the unplug_fn. And swap_list_lock
+ * hold swap_lock while calling the unplug_fn. And swap_lock
  * cannot be turned into a semaphore.
  */
 static DECLARE_RWSEM(swap_unplug_sem);
@@ -105,7 +105,7 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
                 si->cluster_nr = SWAPFILE_CLUSTER - 1;
                 if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER)
                         goto lowest;
-                swap_device_unlock(si);
+                spin_unlock(&swap_lock);
 
                 offset = si->lowest_bit;
                 last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
@@ -115,7 +115,7 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
                         if (si->swap_map[offset])
                                 last_in_cluster = offset + SWAPFILE_CLUSTER;
                         else if (offset == last_in_cluster) {
-                                swap_device_lock(si);
+                                spin_lock(&swap_lock);
                                 si->cluster_next = offset-SWAPFILE_CLUSTER-1;
                                 goto cluster;
                         }
@@ -124,7 +124,7 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
                                 latency_ration = LATENCY_LIMIT;
                         }
                 }
-                swap_device_lock(si);
+                spin_lock(&swap_lock);
                 goto lowest;
         }
 
@@ -153,10 +153,10 @@ checks:	if (!(si->flags & SWP_WRITEOK))
                 return offset;
         }
 
-        swap_device_unlock(si);
+        spin_unlock(&swap_lock);
         while (++offset <= si->highest_bit) {
                 if (!si->swap_map[offset]) {
-                        swap_device_lock(si);
+                        spin_lock(&swap_lock);
                         goto checks;
                 }
                 if (unlikely(--latency_ration < 0)) {
@@ -164,7 +164,7 @@ checks:	if (!(si->flags & SWP_WRITEOK))
                         latency_ration = LATENCY_LIMIT;
                 }
         }
-        swap_device_lock(si);
+        spin_lock(&swap_lock);
         goto lowest;
 
 no_page:
@@ -179,7 +179,7 @@ swp_entry_t get_swap_page(void)
         int type, next;
         int wrapped = 0;
 
-        swap_list_lock();
+        spin_lock(&swap_lock);
         if (nr_swap_pages <= 0)
                 goto noswap;
         nr_swap_pages--;
@@ -199,19 +199,17 @@ swp_entry_t get_swap_page(void)
                         continue;
 
                 swap_list.next = next;
-                swap_device_lock(si);
-                swap_list_unlock();
                 offset = scan_swap_map(si);
-                swap_device_unlock(si);
-                if (offset)
+                if (offset) {
+                        spin_unlock(&swap_lock);
                         return swp_entry(type, offset);
-                swap_list_lock();
+                }
                 next = swap_list.next;
         }
 
         nr_swap_pages++;
 noswap:
-        swap_list_unlock();
+        spin_unlock(&swap_lock);
         return (swp_entry_t) {0};
 }
 
@@ -233,8 +231,7 @@ static struct swap_info_struct * swap_info_get(swp_entry_t entry)
                 goto bad_offset;
         if (!p->swap_map[offset])
                 goto bad_free;
-        swap_list_lock();
-        swap_device_lock(p);
+        spin_lock(&swap_lock);
         return p;
 
 bad_free:
@@ -252,12 +249,6 @@ out:
         return NULL;
 }       
 
-static void swap_info_put(struct swap_info_struct * p)
-{
-        swap_device_unlock(p);
-        swap_list_unlock();
-}
-
 static int swap_entry_free(struct swap_info_struct *p, unsigned long offset)
 {
         int count = p->swap_map[offset];
@@ -290,7 +281,7 @@ void swap_free(swp_entry_t entry)
         p = swap_info_get(entry);
         if (p) {
                 swap_entry_free(p, swp_offset(entry));
-                swap_info_put(p);
+                spin_unlock(&swap_lock);
         }
 }
 
@@ -308,7 +299,7 @@ static inline int page_swapcount(struct page *page)
         if (p) {
                 /* Subtract the 1 for the swap cache itself */
                 count = p->swap_map[swp_offset(entry)] - 1;
-                swap_info_put(p);
+                spin_unlock(&swap_lock);
         }
         return count;
 }
@@ -365,7 +356,7 @@ int remove_exclusive_swap_page(struct page *page)
                 }
                 write_unlock_irq(&swapper_space.tree_lock);
         }
-        swap_info_put(p);
+        spin_unlock(&swap_lock);
 
         if (retval) {
                 swap_free(entry);
@@ -388,7 +379,7 @@ void free_swap_and_cache(swp_entry_t entry)
         if (p) {
                 if (swap_entry_free(p, swp_offset(entry)) == 1)
                         page = find_trylock_page(&swapper_space, entry.val);
-                swap_info_put(p);
+                spin_unlock(&swap_lock);
         }
         if (page) {
                 int one_user;
@@ -558,10 +549,10 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si,
         int count;
 
         /*
-         * No need for swap_device_lock(si) here: we're just looking
+         * No need for swap_lock here: we're just looking
          * for whether an entry is in use, not modifying it; false
          * hits are okay, and sys_swapoff() has already prevented new
-         * allocations from this area (while holding swap_list_lock()).
+         * allocations from this area (while holding swap_lock).
          */
         for (;;) {
                 if (++i >= max) {
@@ -751,9 +742,9 @@ static int try_to_unuse(unsigned int type)
                  * report them; but do report if we reset SWAP_MAP_MAX.
                  */
                 if (*swap_map == SWAP_MAP_MAX) {
-                        swap_device_lock(si);
+                        spin_lock(&swap_lock);
                         *swap_map = 1;
-                        swap_device_unlock(si);
+                        spin_unlock(&swap_lock);
                         reset_overflow = 1;
                 }
 
@@ -817,9 +808,9 @@ static int try_to_unuse(unsigned int type)
 }
 
 /*
- * After a successful try_to_unuse, if no swap is now in use, we know we
- * can empty the mmlist.  swap_list_lock must be held on entry and exit.
- * Note that mmlist_lock nests inside swap_list_lock, and an mm must be
+ * After a successful try_to_unuse, if no swap is now in use, we know
+ * we can empty the mmlist.  swap_lock must be held on entry and exit.
+ * Note that mmlist_lock nests inside swap_lock, and an mm must be
  * added to the mmlist just after page_duplicate - before would be racy.
  */
 static void drain_mmlist(void)
@@ -1092,7 +1083,7 @@ asmlinkage long sys_swapoff(const char __user * specialfile)
 
         mapping = victim->f_mapping;
         prev = -1;
-        swap_list_lock();
+        spin_lock(&swap_lock);
         for (type = swap_list.head; type >= 0; type = swap_info[type].next) {
                 p = swap_info + type;
                 if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
@@ -1103,14 +1094,14 @@ asmlinkage long sys_swapoff(const char __user * specialfile)
         }
         if (type < 0) {
                 err = -EINVAL;
-                swap_list_unlock();
+                spin_unlock(&swap_lock);
                 goto out_dput;
         }
         if (!security_vm_enough_memory(p->pages))
                 vm_unacct_memory(p->pages);
         else {
                 err = -ENOMEM;
-                swap_list_unlock();
+                spin_unlock(&swap_lock);
                 goto out_dput;
         }
         if (prev < 0) {
@@ -1124,10 +1115,8 @@ 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();
+        spin_unlock(&swap_lock);
 
         current->flags |= PF_SWAPOFF;
         err = try_to_unuse(type);
@@ -1135,7 +1124,7 @@ asmlinkage long sys_swapoff(const char __user * specialfile)
 
         if (err) {
                 /* re-insert swap space back into swap_list */
-                swap_list_lock();
+                spin_lock(&swap_lock);
                 for (prev = -1, i = swap_list.head; i >= 0; prev = i, i = swap_info[i].next)
                         if (p->prio >= swap_info[i].prio)
                                 break;
@@ -1146,10 +1135,8 @@ asmlinkage long sys_swapoff(const char __user * specialfile)
                         swap_info[prev].next = p - swap_info;
                 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();
+                spin_unlock(&swap_lock);
                 goto out_dput;
         }
 
@@ -1157,30 +1144,27 @@ asmlinkage long sys_swapoff(const char __user * specialfile)
         down_write(&swap_unplug_sem);
         up_write(&swap_unplug_sem);
 
+        destroy_swap_extents(p);
+        down(&swapon_sem);
+        spin_lock(&swap_lock);
+        drain_mmlist();
+
         /* wait for anyone still in scan_swap_map */
-        swap_device_lock(p);
         p->highest_bit = 0;             /* cuts scans short */
         while (p->flags >= SWP_SCANNING) {
-                swap_device_unlock(p);
+                spin_unlock(&swap_lock);
                 set_current_state(TASK_UNINTERRUPTIBLE);
                 schedule_timeout(1);
-                swap_device_lock(p);
+                spin_lock(&swap_lock);
         }
-        swap_device_unlock(p);
 
-        destroy_swap_extents(p);
-        down(&swapon_sem);
-        swap_list_lock();
-        drain_mmlist();
-        swap_device_lock(p);
         swap_file = p->swap_file;
         p->swap_file = NULL;
         p->max = 0;
         swap_map = p->swap_map;
         p->swap_map = NULL;
         p->flags = 0;
-        swap_device_unlock(p);
-        swap_list_unlock();
+        spin_unlock(&swap_lock);
         up(&swapon_sem);
         vfree(swap_map);
         inode = mapping->host;
@@ -1324,7 +1308,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;
-        swap_list_lock();
+        spin_lock(&swap_lock);
         p = swap_info;
         for (type = 0 ; type < nr_swapfiles ; type++,p++)
                 if (!(p->flags & SWP_USED))
@@ -1343,7 +1327,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
          * swp_entry_t or the architecture definition of a swap pte.
          */
         if (type > swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))) {
-                swap_list_unlock();
+                spin_unlock(&swap_lock);
                 goto out;
         }
         if (type >= nr_swapfiles)
@@ -1357,7 +1341,6 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
         p->highest_bit = 0;
         p->cluster_nr = 0;
         p->inuse_pages = 0;
-        spin_lock_init(&p->sdev_lock);
         p->next = -1;
         if (swap_flags & SWAP_FLAG_PREFER) {
                 p->prio =
@@ -1365,7 +1348,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
         } else {
                 p->prio = --least_priority;
         }
-        swap_list_unlock();
+        spin_unlock(&swap_lock);
         name = getname(specialfile);
         error = PTR_ERR(name);
         if (IS_ERR(name)) {
@@ -1542,8 +1525,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
         }
 
         down(&swapon_sem);
-        swap_list_lock();
-        swap_device_lock(p);
+        spin_lock(&swap_lock);
         p->flags = SWP_ACTIVE;
         nr_swap_pages += nr_good_pages;
         total_swap_pages += nr_good_pages;
@@ -1567,8 +1549,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
         } else {
                 swap_info[prev].next = p - swap_info;
         }
-        swap_device_unlock(p);
-        swap_list_unlock();
+        spin_unlock(&swap_lock);
         up(&swapon_sem);
         error = 0;
         goto out;
@@ -1579,14 +1560,14 @@ bad_swap:
         }
         destroy_swap_extents(p);
 bad_swap_2:
-        swap_list_lock();
+        spin_lock(&swap_lock);
         swap_map = p->swap_map;
         p->swap_file = NULL;
         p->swap_map = NULL;
         p->flags = 0;
         if (!(swap_flags & SWAP_FLAG_PREFER))
                 ++least_priority;
-        swap_list_unlock();
+        spin_unlock(&swap_lock);
         vfree(swap_map);
         if (swap_file)
                 filp_close(swap_file, NULL);
@@ -1610,7 +1591,7 @@ void si_swapinfo(struct sysinfo *val)
         unsigned int i;
         unsigned long nr_to_be_unused = 0;
 
-        swap_list_lock();
+        spin_lock(&swap_lock);
         for (i = 0; i < nr_swapfiles; i++) {
                 if (!(swap_info[i].flags & SWP_USED) ||
                      (swap_info[i].flags & SWP_WRITEOK))
@@ -1619,7 +1600,7 @@ void si_swapinfo(struct sysinfo *val)
         }
         val->freeswap = nr_swap_pages + nr_to_be_unused;
         val->totalswap = total_swap_pages + nr_to_be_unused;
-        swap_list_unlock();
+        spin_unlock(&swap_lock);
 }
 
 /*
@@ -1640,7 +1621,7 @@ int swap_duplicate(swp_entry_t entry)
         p = type + swap_info;
         offset = swp_offset(entry);
 
-        swap_device_lock(p);
+        spin_lock(&swap_lock);
         if (offset < p->max && p->swap_map[offset]) {
                 if (p->swap_map[offset] < SWAP_MAP_MAX - 1) {
                         p->swap_map[offset]++;
@@ -1652,7 +1633,7 @@ int swap_duplicate(swp_entry_t entry)
                         result = 1;
                 }
         }
-        swap_device_unlock(p);
+        spin_unlock(&swap_lock);
 out:
         return result;
 
@@ -1668,7 +1649,7 @@ get_swap_info_struct(unsigned type)
 }
 
 /*
- * swap_device_lock prevents swap_map being freed. Don't grab an extra
+ * swap_lock prevents swap_map being freed. Don't grab an extra
  * reference on the swaphandle, it doesn't matter if it becomes unused.
  */
 int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
@@ -1684,7 +1665,7 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
                 toff++, i--;
         *offset = toff;
 
-        swap_device_lock(swapdev);
+        spin_lock(&swap_lock);
         do {
                 /* Don't read-ahead past the end of the swap area */
                 if (toff >= swapdev->max)
@@ -1697,6 +1678,6 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
                 toff++;
                 ret++;
         } while (--i);
-        swap_device_unlock(swapdev);
+        spin_unlock(&swap_lock);
         return ret;
 }