1 files changed, 250 insertions, 54 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index c0d7b9ed0c16..cc5e7ebf2d2c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -35,11 +35,14 @@
 #include <linux/swapops.h>
 #include <linux/page_cgroup.h>
 
+static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
+                                 unsigned char);
+static void free_swap_count_continuations(struct swap_info_struct *);
+
 static DEFINE_SPINLOCK(swap_lock);
 static unsigned int nr_swapfiles;
 long nr_swap_pages;
 long total_swap_pages;
-static int swap_overflow;
 static int least_priority;
 
 static const char Bad_file[] = "Bad swap file entry ";
@@ -55,7 +58,7 @@ static DEFINE_MUTEX(swapon_mutex);
 
 static inline unsigned char swap_count(unsigned char ent)
 {
-        return ent & ~SWAP_HAS_CACHE;
+        return ent & ~SWAP_HAS_CACHE;   /* may include SWAP_HAS_CONT flag */
 }
 
 /* returns 1 if swap entry is freed */
@@ -545,8 +548,15 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
         if (usage == SWAP_HAS_CACHE) {
                 VM_BUG_ON(!has_cache);
                 has_cache = 0;
-        } else if (count < SWAP_MAP_MAX)
-                count--;
+        } else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
+                if (count == COUNT_CONTINUED) {
+                        if (swap_count_continued(p, offset, count))
+                                count = SWAP_MAP_MAX | COUNT_CONTINUED;
+                        else
+                                count = SWAP_MAP_MAX;
+                } else
+                        count--;
+        }
 
         if (!count)
                 mem_cgroup_uncharge_swap(entry);
@@ -604,6 +614,8 @@ void swapcache_free(swp_entry_t entry, struct page *page)
 
 /*
  * How many references to page are currently swapped out?
+ * This does not give an exact answer when swap count is continued,
+ * but does include the high COUNT_CONTINUED flag to allow for that.
  */
 static inline int page_swapcount(struct page *page)
 {
@@ -1019,7 +1031,6 @@ static int try_to_unuse(unsigned int type)
         swp_entry_t entry;
         unsigned int i = 0;
         int retval = 0;
-        int reset_overflow = 0;
         int shmem;
 
         /*
@@ -1034,8 +1045,7 @@ static int try_to_unuse(unsigned int type)
          * together, child after parent.  If we race with dup_mmap(), we
          * prefer to resolve parent before child, lest we miss entries
          * duplicated after we scanned child: using last mm would invert
-         * that.  Though it's only a serious concern when an overflowed
-         * swap count is reset from SWAP_MAP_MAX, preventing a rescan.
+         * that.
          */
         start_mm = &init_mm;
         atomic_inc(&init_mm.mm_users);
@@ -1165,36 +1175,6 @@ static int try_to_unuse(unsigned int type)
                 }
 
                 /*
-                 * 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.
-                 *
-                 * Yes, that's wrong: though very unlikely, swap count 0x7ffe
-                 * could surely occur if pid_max raised from PID_MAX_DEFAULT;
-                 * and we are now lowering SWAP_MAP_MAX to 0x7e, making it
-                 * much easier to reach.  But the next patch will fix that.
-                 *
-                 * We know "Undead"s can happen, they're okay, so don't
-                 * report them; but do report if we reset 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 = SWAP_HAS_CACHE;
-                        spin_unlock(&swap_lock);
-                        reset_overflow = 1;
-                }
-
-                /*
                  * If a reference remains (rare), we would like to leave
                  * the page in the swap cache; but try_to_unmap could
                  * then re-duplicate the entry once we drop page lock,
@@ -1235,7 +1215,6 @@ 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);
 
@@ -1247,10 +1226,6 @@ retry:
         }
 
         mmput(start_mm);
-        if (reset_overflow) {
-                printk(KERN_WARNING "swapoff: cleared swap entry overflow\n");
-                swap_overflow = 0;
-        }
         return retval;
 }
 
@@ -1593,6 +1568,9 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
         up_write(&swap_unplug_sem);
 
         destroy_swap_extents(p);
+        if (p->flags & SWP_CONTINUED)
+                free_swap_count_continuations(p);
+
         mutex_lock(&swapon_mutex);
         spin_lock(&swap_lock);
         drain_mmlist();
@@ -2079,14 +2057,13 @@ void si_swapinfo(struct sysinfo *val)
 /*
  * Verify that a swap entry is valid and increment its swap map count.
  *
- * 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
+ * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
  */
 static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 {
@@ -2126,15 +2103,14 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 
         } else if (count || has_cache) {
 
-                if (count < SWAP_MAP_MAX - 1)
-                        count++;
-                else if (count <= SWAP_MAP_MAX) {
-                        if (swap_overflow++ < 5)
-                                printk(KERN_WARNING
-                                       "swap_dup: swap entry overflow\n");
-                        count = SWAP_MAP_MAX;
-                } else
+                if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
+                        count += usage;
+                else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
                         err = -EINVAL;
+                else if (swap_count_continued(p, offset, count))
+                        count = COUNT_CONTINUED;
+                else
+                        err = -ENOMEM;
         } else
                 err = -ENOENT;                  /* unused swap entry */
 
@@ -2153,9 +2129,13 @@ bad_file:
 /*
  * increase reference count of swap entry by 1.
  */
-void swap_duplicate(swp_entry_t entry)
+int swap_duplicate(swp_entry_t entry)
 {
-        __swap_duplicate(entry, 1);
+        int err = 0;
+
+        while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
+                err = add_swap_count_continuation(entry, GFP_ATOMIC);
+        return err;
 }
 
 /*
@@ -2222,3 +2202,219 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
         *offset = ++toff;
         return nr_pages? ++nr_pages: 0;
 }
+
+/*
+ * add_swap_count_continuation - called when a swap count is duplicated
+ * beyond SWAP_MAP_MAX, it allocates a new page and links that to the entry's
+ * page of the original vmalloc'ed swap_map, to hold the continuation count
+ * (for that entry and for its neighbouring PAGE_SIZE swap entries).  Called
+ * again when count is duplicated beyond SWAP_MAP_MAX * SWAP_CONT_MAX, etc.
+ *
+ * These continuation pages are seldom referenced: the common paths all work
+ * on the original swap_map, only referring to a continuation page when the
+ * low "digit" of a count is incremented or decremented through SWAP_MAP_MAX.
+ *
+ * add_swap_count_continuation(, GFP_ATOMIC) can be called while holding
+ * page table locks; if it fails, add_swap_count_continuation(, GFP_KERNEL)
+ * can be called after dropping locks.
+ */
+int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
+{
+        struct swap_info_struct *si;
+        struct page *head;
+        struct page *page;
+        struct page *list_page;
+        pgoff_t offset;
+        unsigned char count;
+
+        /*
+         * When debugging, it's easier to use __GFP_ZERO here; but it's better
+         * for latency not to zero a page while GFP_ATOMIC and holding locks.
+         */
+        page = alloc_page(gfp_mask | __GFP_HIGHMEM);
+
+        si = swap_info_get(entry);
+        if (!si) {
+                /*
+                 * An acceptable race has occurred since the failing
+                 * __swap_duplicate(): the swap entry has been freed,
+                 * perhaps even the whole swap_map cleared for swapoff.
+                 */
+                goto outer;
+        }
+
+        offset = swp_offset(entry);
+        count = si->swap_map[offset] & ~SWAP_HAS_CACHE;
+
+        if ((count & ~COUNT_CONTINUED) != SWAP_MAP_MAX) {
+                /*
+                 * The higher the swap count, the more likely it is that tasks
+                 * will race to add swap count continuation: we need to avoid
+                 * over-provisioning.
+                 */
+                goto out;
+        }
+
+        if (!page) {
+                spin_unlock(&swap_lock);
+                return -ENOMEM;
+        }
+
+        /*
+         * We are fortunate that although vmalloc_to_page uses pte_offset_map,
+         * no architecture is using highmem pages for kernel pagetables: so it
+         * will not corrupt the GFP_ATOMIC caller's atomic pagetable kmaps.
+         */
+        head = vmalloc_to_page(si->swap_map + offset);
+        offset &= ~PAGE_MASK;
+
+        /*
+         * Page allocation does not initialize the page's lru field,
+         * but it does always reset its private field.
+         */
+        if (!page_private(head)) {
+                BUG_ON(count & COUNT_CONTINUED);
+                INIT_LIST_HEAD(&head->lru);
+                set_page_private(head, SWP_CONTINUED);
+                si->flags |= SWP_CONTINUED;
+        }
+
+        list_for_each_entry(list_page, &head->lru, lru) {
+                unsigned char *map;
+
+                /*
+                 * If the previous map said no continuation, but we've found
+                 * a continuation page, free our allocation and use this one.
+                 */
+                if (!(count & COUNT_CONTINUED))
+                        goto out;
+
+                map = kmap_atomic(list_page, KM_USER0) + offset;
+                count = *map;
+                kunmap_atomic(map, KM_USER0);
+
+                /*
+                 * If this continuation count now has some space in it,
+                 * free our allocation and use this one.
+                 */
+                if ((count & ~COUNT_CONTINUED) != SWAP_CONT_MAX)
+                        goto out;
+        }
+
+        list_add_tail(&page->lru, &head->lru);
+        page = NULL;                    /* now it's attached, don't free it */
+out:
+        spin_unlock(&swap_lock);
+outer:
+        if (page)
+                __free_page(page);
+        return 0;
+}
+
+/*
+ * swap_count_continued - when the original swap_map count is incremented
+ * from SWAP_MAP_MAX, check if there is already a continuation page to carry
+ * into, carry if so, or else fail until a new continuation page is allocated;
+ * when the original swap_map count is decremented from 0 with continuation,
+ * borrow from the continuation and report whether it still holds more.
+ * Called while __swap_duplicate() or swap_entry_free() holds swap_lock.
+ */
+static bool swap_count_continued(struct swap_info_struct *si,
+                                 pgoff_t offset, unsigned char count)
+{
+        struct page *head;
+        struct page *page;
+        unsigned char *map;
+
+        head = vmalloc_to_page(si->swap_map + offset);
+        if (page_private(head) != SWP_CONTINUED) {
+                BUG_ON(count & COUNT_CONTINUED);
+                return false;           /* need to add count continuation */
+        }
+
+        offset &= ~PAGE_MASK;
+        page = list_entry(head->lru.next, struct page, lru);
+        map = kmap_atomic(page, KM_USER0) + offset;
+
+        if (count == SWAP_MAP_MAX)      /* initial increment from swap_map */
+                goto init_map;          /* jump over SWAP_CONT_MAX checks */
+
+        if (count == (SWAP_MAP_MAX | COUNT_CONTINUED)) { /* incrementing */
+                /*
+                 * Think of how you add 1 to 999
+                 */
+                while (*map == (SWAP_CONT_MAX | COUNT_CONTINUED)) {
+                        kunmap_atomic(map, KM_USER0);
+                        page = list_entry(page->lru.next, struct page, lru);
+                        BUG_ON(page == head);
+                        map = kmap_atomic(page, KM_USER0) + offset;
+                }
+                if (*map == SWAP_CONT_MAX) {
+                        kunmap_atomic(map, KM_USER0);
+                        page = list_entry(page->lru.next, struct page, lru);
+                        if (page == head)
+                                return false;   /* add count continuation */
+                        map = kmap_atomic(page, KM_USER0) + offset;
+init_map:               *map = 0;               /* we didn't zero the page */
+                }
+                *map += 1;
+                kunmap_atomic(map, KM_USER0);
+                page = list_entry(page->lru.prev, struct page, lru);
+                while (page != head) {
+                        map = kmap_atomic(page, KM_USER0) + offset;
+                        *map = COUNT_CONTINUED;
+                        kunmap_atomic(map, KM_USER0);
+                        page = list_entry(page->lru.prev, struct page, lru);
+                }
+                return true;                    /* incremented */
+
+        } else {                                /* decrementing */
+                /*
+                 * Think of how you subtract 1 from 1000
+                 */
+                BUG_ON(count != COUNT_CONTINUED);
+                while (*map == COUNT_CONTINUED) {
+                        kunmap_atomic(map, KM_USER0);
+                        page = list_entry(page->lru.next, struct page, lru);
+                        BUG_ON(page == head);
+                        map = kmap_atomic(page, KM_USER0) + offset;
+                }
+                BUG_ON(*map == 0);
+                *map -= 1;
+                if (*map == 0)
+                        count = 0;
+                kunmap_atomic(map, KM_USER0);
+                page = list_entry(page->lru.prev, struct page, lru);
+                while (page != head) {
+                        map = kmap_atomic(page, KM_USER0) + offset;
+                        *map = SWAP_CONT_MAX | count;
+                        count = COUNT_CONTINUED;
+                        kunmap_atomic(map, KM_USER0);
+                        page = list_entry(page->lru.prev, struct page, lru);
+                }
+                return count == COUNT_CONTINUED;
+        }
+}
+
+/*
+ * free_swap_count_continuations - swapoff free all the continuation pages
+ * appended to the swap_map, after swap_map is quiesced, before vfree'ing it.
+ */
+static void free_swap_count_continuations(struct swap_info_struct *si)
+{
+        pgoff_t offset;
+
+        for (offset = 0; offset < si->max; offset += PAGE_SIZE) {
+                struct page *head;
+                head = vmalloc_to_page(si->swap_map + offset);
+                if (page_private(head)) {
+                        struct list_head *this, *next;
+                        list_for_each_safe(this, next, &head->lru) {
+                                struct page *page;
+                                page = list_entry(this, struct page, lru);
+                                list_del(this);
+                                __free_page(page);
+                        }
+                }
+        }
+}