mm: add_to_swap_cache() must not sleep

After commit 355cfa73 ("mm: modify swap_map and add SWAP_HAS_CACHE flag"),
read_swap_cache_async() will busy-wait while a entry doesn't exist in swap
cache but it has SWAP_HAS_CACHE flag.

Such entries can exist on add/delete path of swap cache.  On add path,
add_to_swap_cache() is called soon after SWAP_HAS_CACHE flag is set, and
on delete path, swapcache_free() will be called (SWAP_HAS_CACHE flag is
cleared) soon after __delete_from_swap_cache() is called.  So, the
busy-wait works well in most cases.

But this mechanism can cause soft lockup if add_to_swap_cache() sleeps and
read_swap_cache_async() tries to swap-in the same entry on the same cpu.

This patch calls radix_tree_preload() before swapcache_prepare() and
divides add_to_swap_cache() into two part: radix_tree_preload() part and
radix_tree_insert() part(define it as __add_to_swap_cache()).

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: 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>

1 files changed, 46 insertions, 24 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 5ae6b8b78c80..b076a1a5a0aa 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -67,10 +67,10 @@ void show_swap_cache_info(void)
 }
 
 /*
- * add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
+ * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
  * but sets SwapCache flag and private instead of mapping and index.
  */
-int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
+static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
 {
         int error;
 
@@ -78,28 +78,37 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
         VM_BUG_ON(PageSwapCache(page));
         VM_BUG_ON(!PageSwapBacked(page));
 
+        page_cache_get(page);
+        SetPageSwapCache(page);
+        set_page_private(page, entry.val);
+
+        spin_lock_irq(&swapper_space.tree_lock);
+        error = radix_tree_insert(&swapper_space.page_tree, entry.val, page);
+        if (likely(!error)) {
+                total_swapcache_pages++;
+                __inc_zone_page_state(page, NR_FILE_PAGES);
+                INC_CACHE_INFO(add_total);
+        }
+        spin_unlock_irq(&swapper_space.tree_lock);
+
+        if (unlikely(error)) {
+                set_page_private(page, 0UL);
+                ClearPageSwapCache(page);
+                page_cache_release(page);
+        }
+
+        return error;
+}
+
+
+int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
+{
+        int error;
+
         error = radix_tree_preload(gfp_mask);
         if (!error) {
-                page_cache_get(page);
-                SetPageSwapCache(page);
-                set_page_private(page, entry.val);
-
-                spin_lock_irq(&swapper_space.tree_lock);
-                error = radix_tree_insert(&swapper_space.page_tree,
-                                                entry.val, page);
-                if (likely(!error)) {
-                        total_swapcache_pages++;
-                        __inc_zone_page_state(page, NR_FILE_PAGES);
-                        INC_CACHE_INFO(add_total);
-                }
-                spin_unlock_irq(&swapper_space.tree_lock);
+                error = __add_to_swap_cache(page, entry);
                 radix_tree_preload_end();
-
-                if (unlikely(error)) {
-                        set_page_private(page, 0UL);
-                        ClearPageSwapCache(page);
-                        page_cache_release(page);
-                }
         }
         return error;
 }
@@ -290,13 +299,24 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                 }
 
                 /*
+                 * call radix_tree_preload() while we can wait.
+                 */
+                err = radix_tree_preload(gfp_mask & GFP_KERNEL);
+                if (err)
+                        break;
+
+                /*
                  * Swap entry may have been freed since our caller observed it.
                  */
                 err = swapcache_prepare(entry);
-                if (err == -EEXIST) /* seems racy */
+                if (err == -EEXIST) {   /* seems racy */
+                        radix_tree_preload_end();
                         continue;
-                if (err)           /* swp entry is obsolete ? */
+                }
+                if (err) {              /* swp entry is obsolete ? */
+                        radix_tree_preload_end();
                         break;
+                }
 
                 /*
                  * Associate the page with swap entry in the swap cache.
@@ -308,8 +328,9 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                  */
                 __set_page_locked(new_page);
                 SetPageSwapBacked(new_page);
-                err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);
+                err = __add_to_swap_cache(new_page, entry);
                 if (likely(!err)) {
+                        radix_tree_preload_end();
                         /*
                          * Initiate read into locked page and return.
                          */
@@ -317,6 +338,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                         swap_readpage(new_page);
                         return new_page;
                 }
+                radix_tree_preload_end();
                 ClearPageSwapBacked(new_page);
                 __clear_page_locked(new_page);
                 swapcache_free(entry, NULL);