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>

1 files changed, 12 insertions, 58 deletions

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);
                 }