mm: add_to_swap_cache() does not return -EEXIST

After commit 355cfa73 ("mm: modify swap_map and add SWAP_HAS_CACHE flag"),
only the context which have set SWAP_HAS_CACHE flag by swapcache_prepare()
or get_swap_page() would call add_to_swap_cache().  So add_to_swap_cache()
doesn't return -EEXIST any more.

Even though it doesn't return -EEXIST, it's not good behavior conceptually
to call swapcache_prepare() in the -EEXIST case, because it means clearing
SWAP_HAS_CACHE flag while the entry is on swap cache.

This patch removes redundant codes and comments from callers of it, and
adds VM_BUG_ON() in error path of add_to_swap_cache() and some comments.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 40 insertions, 37 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index bd20f8bb02aa..376d8f0a8f6b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1097,6 +1097,10 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
         shmem_swp_unmap(entry);
 unlock:
         spin_unlock(&info->lock);
+        /*
+         * add_to_swap_cache() doesn't return -EEXIST, so we can safely
+         * clear SWAP_HAS_CACHE flag.
+         */
         swapcache_free(swap, NULL);
 redirty:
         set_page_dirty(page);
diff --git a/mm/swap_state.c b/mm/swap_state.c
index b076a1a5a0aa..6d1daeb1cb4a 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -92,6 +92,12 @@ static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
         spin_unlock_irq(&swapper_space.tree_lock);
 
         if (unlikely(error)) {
+                /*
+                 * Only the context which have set SWAP_HAS_CACHE flag
+                 * would call add_to_swap_cache().
+                 * So add_to_swap_cache() doesn't returns -EEXIST.
+                 */
+                VM_BUG_ON(error == -EEXIST);
                 set_page_private(page, 0UL);
                 ClearPageSwapCache(page);
                 page_cache_release(page);
@@ -146,38 +152,34 @@ int add_to_swap(struct page *page)
         VM_BUG_ON(!PageLocked(page));
         VM_BUG_ON(!PageUptodate(page));
 
-        for (;;) {
-                entry = get_swap_page();
-                if (!entry.val)
-                        return 0;
+        entry = get_swap_page();
+        if (!entry.val)
+                return 0;
 
+        /*
+         * Radix-tree node allocations from PF_MEMALLOC contexts could
+         * completely exhaust the page allocator. __GFP_NOMEMALLOC
+         * stops emergency reserves from being allocated.
+         *
+         * TODO: this could cause a theoretical memory reclaim
+         * deadlock in the swap out path.
+         */
+        /*
+         * Add it to the swap cache and mark it dirty
+         */
+        err = add_to_swap_cache(page, entry,
+                        __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
+
+        if (!err) {     /* Success */
+                SetPageDirty(page);
+                return 1;
+        } else {        /* -ENOMEM radix-tree allocation failure */
                 /*
-                 * Radix-tree node allocations from PF_MEMALLOC contexts could
-                 * completely exhaust the page allocator. __GFP_NOMEMALLOC
-                 * stops emergency reserves from being allocated.
-                 *
-                 * TODO: this could cause a theoretical memory reclaim
-                 * deadlock in the swap out path.
-                 */
-                /*
-                 * Add it to the swap cache and mark it dirty
+                 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
+                 * clear SWAP_HAS_CACHE flag.
                  */
-                err = add_to_swap_cache(page, entry,
-                                __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
-
-                switch (err) {
-                case 0:                         /* Success */
-                        SetPageDirty(page);
-                        return 1;
-                case -EEXIST:
-                        /* Raced with "speculative" read_swap_cache_async */
-                        swapcache_free(entry, NULL);
-                        continue;
-                default:
-                        /* -ENOMEM radix-tree allocation failure */
-                        swapcache_free(entry, NULL);
-                        return 0;
-                }
+                swapcache_free(entry, NULL);
+                return 0;
         }
 }
 
@@ -318,14 +320,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                         break;
                 }
 
-                /*
-                 * Associate the page with swap entry in the swap cache.
-                 * May fail (-EEXIST) if there is already a page associated
-                 * with this entry in the swap cache: added by a racing
-                 * read_swap_cache_async, or add_to_swap or shmem_writepage
-                 * re-using the just freed swap entry for an existing page.
-                 * May fail (-ENOMEM) if radix-tree node allocation failed.
-                 */
+                /* May fail (-ENOMEM) if radix-tree node allocation failed. */
                 __set_page_locked(new_page);
                 SetPageSwapBacked(new_page);
                 err = __add_to_swap_cache(new_page, entry);
@@ -341,6 +336,10 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                 radix_tree_preload_end();
                 ClearPageSwapBacked(new_page);
                 __clear_page_locked(new_page);
+                /*
+                 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
+                 * clear SWAP_HAS_CACHE flag.
+                 */
                 swapcache_free(entry, NULL);
         } while (err != -ENOMEM);