[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>

1 files changed, 2 insertions, 9 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index db3b5de7c92..3c9ff004815 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;