mm: try_to_free_swap replaces remove_exclusive_swap_page

remove_exclusive_swap_page(): its problem is in living up to its name.

It doesn't matter if someone else has a reference to the page (raised
page_count); it doesn't matter if the page is mapped into userspace
(raised page_mapcount - though that hints it may be worth keeping the
swap): all that matters is that there be no more references to the swap
(and no writeback in progress).

swapoff (try_to_unuse) has been removing pages from swapcache for years,
with no concern for page count or page mapcount, and we used to have a
comment in lookup_swap_cache() recognizing that: if you go for a page of
swapcache, you'll get the right page, but it could have been removed from
swapcache by the time you get page lock.

So, give up asking for exclusivity: get rid of
remove_exclusive_swap_page(), and remove_exclusive_swap_page_ref() and
remove_exclusive_swap_page_count() which were spawned for the recent LRU
work: replace them by the simpler try_to_free_swap() which just checks
page_swapcount().

Similarly, remove the page_count limitation from free_swap_and_count(),
but assume that it's worth holding on to the swap if page is mapped and
swap nowhere near full.  Add a vm_swap_full() test in free_swap_cache()?
It would be consistent, but I think we probably have enough for now.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Robin Holt <holt@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

7 files changed, 22 insertions, 75 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 366556c5b148..c3ecd478840e 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -305,8 +305,7 @@ extern sector_t map_swap_page(struct swap_info_struct *, pgoff_t);
 extern sector_t swapdev_block(int, pgoff_t);
 extern struct swap_info_struct *get_swap_info_struct(unsigned);
 extern int reuse_swap_page(struct page *);
-extern int remove_exclusive_swap_page(struct page *);
-extern int remove_exclusive_swap_page_ref(struct page *);
+extern int try_to_free_swap(struct page *);
 struct backing_dev_info;
 
 /* linux/mm/thrash.c */
@@ -388,12 +387,7 @@ static inline void delete_from_swap_cache(struct page *page)
 
 #define reuse_swap_page(page)   (page_mapcount(page) == 1)
 
-static inline int remove_exclusive_swap_page(struct page *p)
-{
-        return 0;
-}
-
-static inline int remove_exclusive_swap_page_ref(struct page *page)
+static inline int try_to_free_swap(struct page *page)
 {
         return 0;
 }
diff --git a/mm/memory.c b/mm/memory.c
index 8f471edcb985..1a83fe5339a9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2403,7 +2403,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
         swap_free(entry);
         if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
-                remove_exclusive_swap_page(page);
+                try_to_free_swap(page);
         unlock_page(page);
 
         if (write_access) {
diff --git a/mm/page_io.c b/mm/page_io.c
index d277a80efa71..dc6ce0afbded 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -98,7 +98,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
         struct bio *bio;
         int ret = 0, rw = WRITE;
 
-        if (remove_exclusive_swap_page(page)) {
+        if (try_to_free_swap(page)) {
                 unlock_page(page);
                 goto out;
         }
diff --git a/mm/swap.c b/mm/swap.c
index ff0b290475fd..ba2c0e8b8b54 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -454,8 +454,7 @@ void pagevec_swap_free(struct pagevec *pvec)
                 struct page *page = pvec->pages[i];
 
                 if (PageSwapCache(page) && trylock_page(page)) {
-                        if (PageSwapCache(page))
-                                remove_exclusive_swap_page_ref(page);
+                        try_to_free_swap(page);
                         unlock_page(page);
                 }
         }
diff --git a/mm/swap_state.c b/mm/swap_state.c
index e793fdea275d..bcb472769299 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -195,14 +195,14 @@ void delete_from_swap_cache(struct page *page)
  * If we are the only user, then try to free up the swap cache. 
  * 
  * Its ok to check for PageSwapCache without the page lock
- * here because we are going to recheck again inside 
- * exclusive_swap_page() _with_ the lock. 
+ * here because we are going to recheck again inside
+ * try_to_free_swap() _with_ the lock.
  *                                      - Marcelo
  */
 static inline void free_swap_cache(struct page *page)
 {
-        if (PageSwapCache(page) && trylock_page(page)) {
-                remove_exclusive_swap_page(page);
+        if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
+                try_to_free_swap(page);
                 unlock_page(page);
         }
 }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index bfd4ee59cb88..f43601827607 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -348,68 +348,23 @@ int reuse_swap_page(struct page *page)
 }
 
 /*
- * Work out if there are any other processes sharing this
- * swap cache page. Free it if you can. Return success.
+ * If swap is getting full, or if there are no more mappings of this page,
+ * then try_to_free_swap is called to free its swap space.
  */
-static int remove_exclusive_swap_page_count(struct page *page, int count)
+int try_to_free_swap(struct page *page)
 {
-        int retval;
-        struct swap_info_struct * p;
-        swp_entry_t entry;
-
         VM_BUG_ON(!PageLocked(page));
 
         if (!PageSwapCache(page))
                 return 0;
         if (PageWriteback(page))
                 return 0;
-        if (page_count(page) != count) /* us + cache + ptes */
-                return 0;
-
-        entry.val = page_private(page);
-        p = swap_info_get(entry);
-        if (!p)
+        if (page_swapcount(page))
                 return 0;
 
-        /* Is the only swap cache user the cache itself? */
-        retval = 0;
-        if (p->swap_map[swp_offset(entry)] == 1) {
-                /* Recheck the page count with the swapcache lock held.. */
-                spin_lock_irq(&swapper_space.tree_lock);
-                if ((page_count(page) == count) && !PageWriteback(page)) {
-                        __delete_from_swap_cache(page);
-                        SetPageDirty(page);
-                        retval = 1;
-                }
-                spin_unlock_irq(&swapper_space.tree_lock);
-        }
-        spin_unlock(&swap_lock);
-
-        if (retval) {
-                swap_free(entry);
-                page_cache_release(page);
-        }
-
-        return retval;
-}
-
-/*
- * Most of the time the page should have two references: one for the
- * process and one for the swap cache.
- */
-int remove_exclusive_swap_page(struct page *page)
-{
-        return remove_exclusive_swap_page_count(page, 2);
-}
-
-/*
- * The pageout code holds an extra reference to the page.  That raises
- * the reference count to test for to 2 for a page that is only in the
- * swap cache plus 1 for each process that maps the page.
- */
-int remove_exclusive_swap_page_ref(struct page *page)
-{
-        return remove_exclusive_swap_page_count(page, 2 + page_mapcount(page));
+        delete_from_swap_cache(page);
+        SetPageDirty(page);
+        return 1;
 }
 
 /*
@@ -436,13 +391,12 @@ void free_swap_and_cache(swp_entry_t entry)
                 spin_unlock(&swap_lock);
         }
         if (page) {
-                int one_user;
-
-                one_user = (page_count(page) == 2);
-                /* Only cache user (+us), or swap space full? Free it! */
-                /* Also recheck PageSwapCache after page is locked (above) */
+                /*
+                 * Not mapped elsewhere, or swap space full? Free it!
+                 * Also recheck PageSwapCache now page is locked (above).
+                 */
                 if (PageSwapCache(page) && !PageWriteback(page) &&
-                                        (one_user || vm_swap_full())) {
+                                (!page_mapped(page) || vm_swap_full())) {
                         delete_from_swap_cache(page);
                         SetPageDirty(page);
                 }
diff --git a/mm/vmscan.c b/mm/vmscan.c
index d196f46c8808..c8601dd36603 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -759,7 +759,7 @@ cull_mlocked:
 activate_locked:
                 /* Not a candidate for swapping, so reclaim swap space. */
                 if (PageSwapCache(page) && vm_swap_full())
-                        remove_exclusive_swap_page_ref(page);
+                        try_to_free_swap(page);
                 VM_BUG_ON(PageActive(page));
                 SetPageActive(page);
                 pgactivate++;