mm: modify swap_map and add SWAP_HAS_CACHE flag

This is a part of the patches for fixing memcg's swap accountinf leak.
But, IMHO, not a bad patch even if no memcg.

There are 2 kinds of references to swap.
 - reference from swap entry
 - reference from swap cache

Then,

 - If there is swap cache && swap's refcnt is 1, there is only swap cache.
  (*) swapcount(entry) == 1 && find_get_page(swapper_space, entry) != NULL

This counting logic have worked well for a long time.  But considering
that we cannot know there is a _real_ reference or not by swap_map[],
current usage of counter is not very good.

This patch adds a flag SWAP_HAS_CACHE and recored information that a swap
entry has a cache or not.  This will remove -1 magic used in swapfile.c
and be a help to avoid unnecessary find_get_page().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Tested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 163 insertions, 56 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 19bdf3017a9e..b9ca029673a5 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -292,7 +292,10 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                 /*
                  * Swap entry may have been freed since our caller observed it.
                  */
-                if (!swapcache_prepare(entry))
+                err = swapcache_prepare(entry);
+                if (err == -EEXIST) /* seems racy */
+                        continue;
+                if (err)           /* swp entry is obsolete ? */
                         break;
 
                 /*
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3187079903fd..0d7296971ad9 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -53,6 +53,33 @@ static struct swap_info_struct swap_info[MAX_SWAPFILES];
 
 static DEFINE_MUTEX(swapon_mutex);
 
+/* For reference count accounting in swap_map */
+/* enum for swap_map[] handling. internal use only */
+enum {
+        SWAP_MAP = 0,   /* ops for reference from swap users */
+        SWAP_CACHE,     /* ops for reference from swap cache */
+};
+
+static inline int swap_count(unsigned short ent)
+{
+        return ent & SWAP_COUNT_MASK;
+}
+
+static inline bool swap_has_cache(unsigned short ent)
+{
+        return !!(ent & SWAP_HAS_CACHE);
+}
+
+static inline unsigned short encode_swapmap(int count, bool has_cache)
+{
+        unsigned short ret = count;
+
+        if (has_cache)
+                return SWAP_HAS_CACHE | ret;
+        return ret;
+}
+
+
 /*
  * We need this because the bdev->unplug_fn can sleep and we cannot
  * hold swap_lock while calling the unplug_fn. And swap_lock
@@ -167,7 +194,8 @@ static int wait_for_discard(void *word)
 #define SWAPFILE_CLUSTER        256
 #define LATENCY_LIMIT           256
 
-static inline unsigned long scan_swap_map(struct swap_info_struct *si)
+static inline unsigned long scan_swap_map(struct swap_info_struct *si,
+                                          int cache)
 {
         unsigned long offset;
         unsigned long scan_base;
@@ -285,7 +313,10 @@ checks:
                 si->lowest_bit = si->max;
                 si->highest_bit = 0;
         }
-        si->swap_map[offset] = 1;
+        if (cache == SWAP_CACHE) /* at usual swap-out via vmscan.c */
+                si->swap_map[offset] = encode_swapmap(0, true);
+        else /* at suspend */
+                si->swap_map[offset] = encode_swapmap(1, false);
         si->cluster_next = offset + 1;
         si->flags -= SWP_SCANNING;
 
@@ -401,7 +432,8 @@ swp_entry_t get_swap_page(void)
                         continue;
 
                 swap_list.next = next;
-                offset = scan_swap_map(si);
+                /* This is called for allocating swap entry for cache */
+                offset = scan_swap_map(si, SWAP_CACHE);
                 if (offset) {
                         spin_unlock(&swap_lock);
                         return swp_entry(type, offset);
@@ -415,6 +447,7 @@ noswap:
         return (swp_entry_t) {0};
 }
 
+/* The only caller of this function is now susupend routine */
 swp_entry_t get_swap_page_of_type(int type)
 {
         struct swap_info_struct *si;
@@ -424,7 +457,8 @@ swp_entry_t get_swap_page_of_type(int type)
         si = swap_info + type;
         if (si->flags & SWP_WRITEOK) {
                 nr_swap_pages--;
-                offset = scan_swap_map(si);
+                /* This is called for allocating swap entry, not cache */
+                offset = scan_swap_map(si, SWAP_MAP);
                 if (offset) {
                         spin_unlock(&swap_lock);
                         return swp_entry(type, offset);
@@ -471,25 +505,38 @@ out:
         return NULL;
 }
 
-static int swap_entry_free(struct swap_info_struct *p, swp_entry_t ent)
+static int swap_entry_free(struct swap_info_struct *p,
+                           swp_entry_t ent, int cache)
 {
         unsigned long offset = swp_offset(ent);
-        int count = p->swap_map[offset];
-
-        if (count < SWAP_MAP_MAX) {
-                count--;
-                p->swap_map[offset] = count;
-                if (!count) {
-                        if (offset < p->lowest_bit)
-                                p->lowest_bit = offset;
-                        if (offset > p->highest_bit)
-                                p->highest_bit = offset;
-                        if (p->prio > swap_info[swap_list.next].prio)
-                                swap_list.next = p - swap_info;
-                        nr_swap_pages++;
-                        p->inuse_pages--;
-                        mem_cgroup_uncharge_swap(ent);
+        int count = swap_count(p->swap_map[offset]);
+        bool has_cache;
+
+        has_cache = swap_has_cache(p->swap_map[offset]);
+
+        if (cache == SWAP_MAP) { /* dropping usage count of swap */
+                if (count < SWAP_MAP_MAX) {
+                        count--;
+                        p->swap_map[offset] = encode_swapmap(count, has_cache);
                 }
+        } else { /* dropping swap cache flag */
+                VM_BUG_ON(!has_cache);
+                p->swap_map[offset] = encode_swapmap(count, false);
+
+        }
+        /* return code. */
+        count = p->swap_map[offset];
+        /* free if no reference */
+        if (!count) {
+                if (offset < p->lowest_bit)
+                        p->lowest_bit = offset;
+                if (offset > p->highest_bit)
+                        p->highest_bit = offset;
+                if (p->prio > swap_info[swap_list.next].prio)
+                        swap_list.next = p - swap_info;
+                nr_swap_pages++;
+                p->inuse_pages--;
+                mem_cgroup_uncharge_swap(ent);
         }
         return count;
 }
@@ -504,7 +551,7 @@ void swap_free(swp_entry_t entry)
 
         p = swap_info_get(entry);
         if (p) {
-                swap_entry_free(p, entry);
+                swap_entry_free(p, entry, SWAP_MAP);
                 spin_unlock(&swap_lock);
         }
 }
@@ -514,9 +561,16 @@ void swap_free(swp_entry_t entry)
  */
 void swapcache_free(swp_entry_t entry, struct page *page)
 {
+        struct swap_info_struct *p;
+
         if (page)
                 mem_cgroup_uncharge_swapcache(page, entry);
-        return swap_free(entry);
+        p = swap_info_get(entry);
+        if (p) {
+                swap_entry_free(p, entry, SWAP_CACHE);
+                spin_unlock(&swap_lock);
+        }
+        return;
 }
 
 /*
@@ -531,8 +585,7 @@ static inline int page_swapcount(struct page *page)
         entry.val = page_private(page);
         p = swap_info_get(entry);
         if (p) {
-                /* Subtract the 1 for the swap cache itself */
-                count = p->swap_map[swp_offset(entry)] - 1;
+                count = swap_count(p->swap_map[swp_offset(entry)]);
                 spin_unlock(&swap_lock);
         }
         return count;
@@ -594,7 +647,7 @@ int free_swap_and_cache(swp_entry_t entry)
 
         p = swap_info_get(entry);
         if (p) {
-                if (swap_entry_free(p, entry) == 1) {
+                if (swap_entry_free(p, entry, SWAP_MAP) == SWAP_HAS_CACHE) {
                         page = find_get_page(&swapper_space, entry.val);
                         if (page && !trylock_page(page)) {
                                 page_cache_release(page);
@@ -901,7 +954,7 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si,
                         i = 1;
                 }
                 count = si->swap_map[i];
-                if (count && count != SWAP_MAP_BAD)
+                if (count && swap_count(count) != SWAP_MAP_BAD)
                         break;
         }
         return i;
@@ -1005,13 +1058,13 @@ static int try_to_unuse(unsigned int type)
                  */
                 shmem = 0;
                 swcount = *swap_map;
-                if (swcount > 1) {
+                if (swap_count(swcount)) {
                         if (start_mm == &init_mm)
                                 shmem = shmem_unuse(entry, page);
                         else
                                 retval = unuse_mm(start_mm, entry, page);
                 }
-                if (*swap_map > 1) {
+                if (swap_count(*swap_map)) {
                         int set_start_mm = (*swap_map >= swcount);
                         struct list_head *p = &start_mm->mmlist;
                         struct mm_struct *new_start_mm = start_mm;
@@ -1021,7 +1074,7 @@ static int try_to_unuse(unsigned int type)
                         atomic_inc(&new_start_mm->mm_users);
                         atomic_inc(&prev_mm->mm_users);
                         spin_lock(&mmlist_lock);
-                        while (*swap_map > 1 && !retval && !shmem &&
+                        while (swap_count(*swap_map) && !retval && !shmem &&
                                         (p = p->next) != &start_mm->mmlist) {
                                 mm = list_entry(p, struct mm_struct, mmlist);
                                 if (!atomic_inc_not_zero(&mm->mm_users))
@@ -1033,14 +1086,16 @@ static int try_to_unuse(unsigned int type)
                                 cond_resched();
 
                                 swcount = *swap_map;
-                                if (swcount <= 1)
+                                if (!swap_count(swcount)) /* any usage ? */
                                         ;
                                 else if (mm == &init_mm) {
                                         set_start_mm = 1;
                                         shmem = shmem_unuse(entry, page);
                                 } else
                                         retval = unuse_mm(mm, entry, page);
-                                if (set_start_mm && *swap_map < swcount) {
+
+                                if (set_start_mm &&
+                                    swap_count(*swap_map) < swcount) {
                                         mmput(new_start_mm);
                                         atomic_inc(&mm->mm_users);
                                         new_start_mm = mm;
@@ -1067,21 +1122,25 @@ static int try_to_unuse(unsigned int type)
                 }
 
                 /*
-                 * How could swap count reach 0x7fff when the maximum
-                 * pid is 0x7fff, and there's no way to repeat a swap
-                 * page within an mm (except in shmem, where it's the
-                 * shared object which takes the reference count)?
-                 * We believe SWAP_MAP_MAX cannot occur in Linux 2.4.
-                 *
+                 * How could swap count reach 0x7ffe ?
+                 * There's no way to repeat a swap page within an mm
+                 * (except in shmem, where it's the shared object which takes
+                 * the reference count)?
+                 * We believe SWAP_MAP_MAX cannot occur.(if occur, unsigned
+                 * short is too small....)
                  * If that's wrong, then we should worry more about
                  * exit_mmap() and do_munmap() cases described above:
                  * we might be resetting SWAP_MAP_MAX too early here.
                  * We know "Undead"s can happen, they're okay, so don't
                  * report them; but do report if we reset SWAP_MAP_MAX.
                  */
-                if (*swap_map == SWAP_MAP_MAX) {
+                /* We might release the lock_page() in unuse_mm(). */
+                if (!PageSwapCache(page) || page_private(page) != entry.val)
+                        goto retry;
+
+                if (swap_count(*swap_map) == SWAP_MAP_MAX) {
                         spin_lock(&swap_lock);
-                        *swap_map = 1;
+                        *swap_map = encode_swapmap(0, true);
                         spin_unlock(&swap_lock);
                         reset_overflow = 1;
                 }
@@ -1099,7 +1158,8 @@ static int try_to_unuse(unsigned int type)
                  * pages would be incorrect if swap supported "shared
                  * private" pages, but they are handled by tmpfs files.
                  */
-                if ((*swap_map > 1) && PageDirty(page) && PageSwapCache(page)) {
+                if (swap_count(*swap_map) &&
+                     PageDirty(page) && PageSwapCache(page)) {
                         struct writeback_control wbc = {
                                 .sync_mode = WB_SYNC_NONE,
                         };
@@ -1126,6 +1186,7 @@ static int try_to_unuse(unsigned int type)
                  * mark page dirty so shrink_page_list will preserve it.
                  */
                 SetPageDirty(page);
+retry:
                 unlock_page(page);
                 page_cache_release(page);
 
@@ -1952,15 +2013,23 @@ void si_swapinfo(struct sysinfo *val)
  *
  * Note: if swap_map[] reaches SWAP_MAP_MAX the entries are treated as
  * "permanent", but will be reclaimed by the next swapoff.
+ * Returns error code in following case.
+ * - success -> 0
+ * - swp_entry is invalid -> EINVAL
+ * - swp_entry is migration entry -> EINVAL
+ * - swap-cache reference is requested but there is already one. -> EEXIST
+ * - swap-cache reference is requested but the entry is not used. -> ENOENT
  */
-int swap_duplicate(swp_entry_t entry)
+static int __swap_duplicate(swp_entry_t entry, bool cache)
 {
         struct swap_info_struct * p;
         unsigned long offset, type;
-        int result = 0;
+        int result = -EINVAL;
+        int count;
+        bool has_cache;
 
         if (is_migration_entry(entry))
-                return 1;
+                return -EINVAL;
 
         type = swp_type(entry);
         if (type >= nr_swapfiles)
@@ -1969,17 +2038,40 @@ int swap_duplicate(swp_entry_t entry)
         offset = swp_offset(entry);
 
         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]++;
-                        result = 1;
-                } else if (p->swap_map[offset] <= SWAP_MAP_MAX) {
+
+        if (unlikely(offset >= p->max))
+                goto unlock_out;
+
+        count = swap_count(p->swap_map[offset]);
+        has_cache = swap_has_cache(p->swap_map[offset]);
+
+        if (cache == SWAP_CACHE) { /* called for swapcache/swapin-readahead */
+
+                /* set SWAP_HAS_CACHE if there is no cache and entry is used */
+                if (!has_cache && count) {
+                        p->swap_map[offset] = encode_swapmap(count, true);
+                        result = 0;
+                } else if (has_cache) /* someone added cache */
+                        result = -EEXIST;
+                else if (!count) /* no users */
+                        result = -ENOENT;
+
+        } else if (count || has_cache) {
+                if (count < SWAP_MAP_MAX - 1) {
+                        p->swap_map[offset] = encode_swapmap(count + 1,
+                                                             has_cache);
+                        result = 0;
+                } else if (count <= SWAP_MAP_MAX) {
                         if (swap_overflow++ < 5)
-                                printk(KERN_WARNING "swap_dup: swap entry overflow\n");
-                        p->swap_map[offset] = SWAP_MAP_MAX;
-                        result = 1;
+                                printk(KERN_WARNING
+                                       "swap_dup: swap entry overflow\n");
+                        p->swap_map[offset] = encode_swapmap(SWAP_MAP_MAX,
+                                                              has_cache);
+                        result = 0;
                 }
-        }
+        } else
+                result = -ENOENT; /* unused swap entry */
+unlock_out:
         spin_unlock(&swap_lock);
 out:
         return result;
@@ -1988,13 +2080,25 @@ bad_file:
         printk(KERN_ERR "swap_dup: %s%08lx\n", Bad_file, entry.val);
         goto out;
 }
+/*
+ * increase reference count of swap entry by 1.
+ */
+void swap_duplicate(swp_entry_t entry)
+{
+        __swap_duplicate(entry, SWAP_MAP);
+}
 
 /*
+ * @entry: swap entry for which we allocate swap cache.
+ *
  * Called when allocating swap cache for exising swap entry,
+ * This can return error codes. Returns 0 at success.
+ * -EBUSY means there is a swap cache.
+ * Note: return code is different from swap_duplicate().
  */
 int swapcache_prepare(swp_entry_t entry)
 {
-        return swap_duplicate(entry);
+        return __swap_duplicate(entry, SWAP_CACHE);
 }
 
 
@@ -2035,7 +2139,7 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
                 /* Don't read in free or bad pages */
                 if (!si->swap_map[toff])
                         break;
-                if (si->swap_map[toff] == SWAP_MAP_BAD)
+                if (swap_count(si->swap_map[toff]) == SWAP_MAP_BAD)
                         break;
         }
         /* Count contiguous allocated slots below our target */
@@ -2043,7 +2147,7 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
                 /* Don't read in free or bad pages */
                 if (!si->swap_map[toff])
                         break;
-                if (si->swap_map[toff] == SWAP_MAP_BAD)
+                if (swap_count(si->swap_map[toff]) == SWAP_MAP_BAD)
                         break;
         }
         spin_unlock(&swap_lock);