frontswap: get rid of swap_lock dependency

Frontswap initialization routine depends on swap_lock, which want to be
atomic about frontswap's first appearance.  IOW, frontswap is not present
and will fail all calls OR frontswap is fully functional but if new
swap_info_struct isn't registered by enable_swap_info, swap subsystem
doesn't start I/O so there is no race between init procedure and page I/O
working on frontswap.

So let's remove unnecessary swap_lock dependency.

Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
[v1: Rebased on my branch, reworked to work with backends loading late]
[v2: Added a check for !map]
[v3: Made the invalidate path follow the init path]
[v4: Address comments by Wanpeng Li <liwanp@linux.vnet.ibm.com>]
Signed-off-by: Konrad Rzeszutek Wilk <konrad@darnok.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Andor Daam <andor.daam@googlemail.com>
Cc: Florian Schmaus <fschmaus@gmail.com>
Cc: Stefan Hengelein <ilendir@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 32 insertions, 16 deletions

diff --git a/mm/frontswap.c b/mm/frontswap.c
index 2760b0f98822..538367ef1372 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -121,8 +121,13 @@ struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
         int i;
 
         for (i = 0; i < MAX_SWAPFILES; i++) {
-                if (test_and_clear_bit(i, need_init))
+                if (test_and_clear_bit(i, need_init)) {
+                        struct swap_info_struct *sis = swap_info[i];
+                        /* __frontswap_init _should_ have set it! */
+                        if (!sis->frontswap_map)
+                                return ERR_PTR(-EINVAL);
                         ops->init(i);
+                }
         }
         /*
          * We MUST have frontswap_ops set _after_ the frontswap_init's
@@ -156,20 +161,30 @@ EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
 /*
  * Called when a swap device is swapon'd.
  */
-void __frontswap_init(unsigned type)
+void __frontswap_init(unsigned type, unsigned long *map)
 {
         struct swap_info_struct *sis = swap_info[type];
 
-        if (frontswap_ops) {
-                BUG_ON(sis == NULL);
-                if (sis->frontswap_map == NULL)
-                        return;
+        BUG_ON(sis == NULL);
+
+        /*
+         * p->frontswap is a bitmap that we MUST have to figure out which page
+         * has gone in frontswap. Without it there is no point of continuing.
+         */
+        if (WARN_ON(!map))
+                return;
+        /*
+         * Irregardless of whether the frontswap backend has been loaded
+         * before this function or it will be later, we _MUST_ have the
+         * p->frontswap set to something valid to work properly.
+         */
+        frontswap_map_set(sis, map);
+        if (frontswap_ops)
                 frontswap_ops->init(type);
-        } else {
+        else {
                 BUG_ON(type > MAX_SWAPFILES);
                 set_bit(type, need_init);
         }
-
 }
 EXPORT_SYMBOL(__frontswap_init);
 
diff --git a/mm/swapfile.c b/mm/swapfile.c
index d417efddfe74..6c340d908b27 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1509,8 +1509,7 @@ static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
 }
 
 static void _enable_swap_info(struct swap_info_struct *p, int prio,
-                                unsigned char *swap_map,
-                                unsigned long *frontswap_map)
+                                unsigned char *swap_map)
 {
         int i, prev;
 
@@ -1519,7 +1518,6 @@ static void _enable_swap_info(struct swap_info_struct *p, int prio,
         else
                 p->prio = --least_priority;
         p->swap_map = swap_map;
-        frontswap_map_set(p, frontswap_map);
         p->flags |= SWP_WRITEOK;
         atomic_long_add(p->pages, &nr_swap_pages);
         total_swap_pages += p->pages;
@@ -1542,10 +1540,10 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
                                 unsigned char *swap_map,
                                 unsigned long *frontswap_map)
 {
+        frontswap_init(p->type, frontswap_map);
         spin_lock(&swap_lock);
         spin_lock(&p->lock);
-        _enable_swap_info(p, prio, swap_map, frontswap_map);
-        frontswap_init(p->type);
+         _enable_swap_info(p, prio, swap_map);
         spin_unlock(&p->lock);
         spin_unlock(&swap_lock);
 }
@@ -1554,7 +1552,7 @@ static void reinsert_swap_info(struct swap_info_struct *p)
 {
         spin_lock(&swap_lock);
         spin_lock(&p->lock);
-        _enable_swap_info(p, p->prio, p->swap_map, frontswap_map_get(p));
+        _enable_swap_info(p, p->prio, p->swap_map);
         spin_unlock(&p->lock);
         spin_unlock(&swap_lock);
 }
@@ -1563,6 +1561,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 {
         struct swap_info_struct *p = NULL;
         unsigned char *swap_map;
+        unsigned long *frontswap_map;
         struct file *swap_file, *victim;
         struct address_space *mapping;
         struct inode *inode;
@@ -1662,12 +1661,14 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
         swap_map = p->swap_map;
         p->swap_map = NULL;
         p->flags = 0;
-        frontswap_invalidate_area(type);
+        frontswap_map = frontswap_map_get(p);
+        frontswap_map_set(p, NULL);
         spin_unlock(&p->lock);
         spin_unlock(&swap_lock);
+        frontswap_invalidate_area(type);
         mutex_unlock(&swapon_mutex);
         vfree(swap_map);
-        vfree(frontswap_map_get(p));
+        vfree(frontswap_map);
         /* Destroy swap account informatin */
         swap_cgroup_swapoff(type);