[cleanup][2/2] mm: add_to_swap_cache() does not return -EEXIST

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

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 a good behavior conceptually
to call swapcache_prepare() in -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>
---
 mm/shmem.c      |    4 +++
 mm/swap_state.c |   75 +++++++++++++++++++++++++++----------------------------
 2 files changed, 41 insertions(+), 38 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index d713239..c71ac6c 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 3e6dd72..e891208 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -96,6 +96,12 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
 		radix_tree_preload_end();
 
 		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);
@@ -137,38 +143,34 @@ 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;
 	}
 }
 
@@ -298,14 +300,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		if (err)           /* swp entry is obsolete ? */
 			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, GFP_ATOMIC);
@@ -319,6 +314,10 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		}
 		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);
 

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