vfs: improve writeback_inodes_wb()

Do not pin/unpin superblock for every inode in writeback_inodes_wb(), pin
it for the whole group of inodes which belong to the same superblock and
call writeback_sb_inodes() handler for them.

Signed-off-by: Edward Shishkin <edward.shishkin@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 76 insertions, 60 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 76fc4d594acb..6841effa47ca 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -553,108 +553,85 @@ select_queue:
         return ret;
 }
 
-static void unpin_sb_for_writeback(struct super_block **psb)
+static void unpin_sb_for_writeback(struct super_block *sb)
 {
-        struct super_block *sb = *psb;
-
-        if (sb) {
-                up_read(&sb->s_umount);
-                put_super(sb);
-                *psb = NULL;
-        }
+        up_read(&sb->s_umount);
+        put_super(sb);
 }
 
+enum sb_pin_state {
+        SB_PINNED,
+        SB_NOT_PINNED,
+        SB_PIN_FAILED
+};
+
 /*
  * For WB_SYNC_NONE writeback, the caller does not have the sb pinned
  * before calling writeback. So make sure that we do pin it, so it doesn't
  * go away while we are writing inodes from it.
- *
- * Returns 0 if the super was successfully pinned (or pinning wasn't needed),
- * 1 if we failed.
  */
-static int pin_sb_for_writeback(struct writeback_control *wbc,
-                                struct inode *inode, struct super_block **psb)
+static enum sb_pin_state pin_sb_for_writeback(struct writeback_control *wbc,
+                                              struct super_block *sb)
 {
-        struct super_block *sb = inode->i_sb;
-
-        /*
-         * If this sb is already pinned, nothing more to do. If not and
-         * *psb is non-NULL, unpin the old one first
-         */
-        if (sb == *psb)
-                return 0;
-        else if (*psb)
-                unpin_sb_for_writeback(psb);
-
         /*
          * Caller must already hold the ref for this
          */
         if (wbc->sync_mode == WB_SYNC_ALL) {
                 WARN_ON(!rwsem_is_locked(&sb->s_umount));
-                return 0;
+                return SB_NOT_PINNED;
         }
-
         spin_lock(&sb_lock);
         sb->s_count++;
         if (down_read_trylock(&sb->s_umount)) {
                 if (sb->s_root) {
                         spin_unlock(&sb_lock);
-                        goto pinned;
+                        return SB_PINNED;
                 }
                 /*
                  * umounted, drop rwsem again and fall through to failure
                  */
                 up_read(&sb->s_umount);
         }
-
         sb->s_count--;
         spin_unlock(&sb_lock);
-        return 1;
-pinned:
-        *psb = sb;
-        return 0;
+        return SB_PIN_FAILED;
 }
 
-static void writeback_inodes_wb(struct bdi_writeback *wb,
-                                struct writeback_control *wbc)
+/*
+ * Write a portion of b_io inodes which belong to @sb.
+ * If @wbc->sb != NULL, then find and write all such
+ * inodes. Otherwise write only ones which go sequentially
+ * in reverse order.
+ * Return 1, if the caller writeback routine should be
+ * interrupted. Otherwise return 0.
+ */
+static int writeback_sb_inodes(struct super_block *sb,
+                               struct bdi_writeback *wb,
+                               struct writeback_control *wbc)
 {
-        struct super_block *sb = wbc->sb, *pin_sb = NULL;
-        const unsigned long start = jiffies;    /* livelock avoidance */
-
-        spin_lock(&inode_lock);
-
-        if (!wbc->for_kupdate || list_empty(&wb->b_io))
-                queue_io(wb, wbc->older_than_this);
-
         while (!list_empty(&wb->b_io)) {
-                struct inode *inode = list_entry(wb->b_io.prev,
-                                                struct inode, i_list);
                 long pages_skipped;
-
-                /*
-                 * super block given and doesn't match, skip this inode
-                 */
-                if (sb && sb != inode->i_sb) {
+                struct inode *inode = list_entry(wb->b_io.prev,
+                                                 struct inode, i_list);
+                if (wbc->sb && sb != inode->i_sb) {
+                        /* super block given and doesn't
+                           match, skip this inode */
                         redirty_tail(inode);
                         continue;
                 }
-
+                if (sb != inode->i_sb)
+                        /* finish with this superblock */
+                        return 0;
                 if (inode->i_state & (I_NEW | I_WILL_FREE)) {
                         requeue_io(inode);
                         continue;
                 }
-
                 /*
                  * Was this inode dirtied after sync_sb_inodes was called?
                  * This keeps sync from extra jobs and livelock.
                  */
-                if (inode_dirtied_after(inode, start))
-                        break;
-
-                if (pin_sb_for_writeback(wbc, inode, &pin_sb)) {
-                        requeue_io(inode);
-                        continue;
-                }
+                if (inode_dirtied_after(inode, wbc->wb_start))
+                        return 1;
 
                 BUG_ON(inode->i_state & (I_FREEING | I_CLEAR));
                 __iget(inode);
@@ -673,14 +650,50 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
                 spin_lock(&inode_lock);
                 if (wbc->nr_to_write <= 0) {
                         wbc->more_io = 1;
-                        break;
+                        return 1;
                 }
                 if (!list_empty(&wb->b_more_io))
                         wbc->more_io = 1;
         }
+        /* b_io is empty */
+        return 1;
+}
+
+static void writeback_inodes_wb(struct bdi_writeback *wb,
+                                struct writeback_control *wbc)
+{
+        int ret = 0;
 
-        unpin_sb_for_writeback(&pin_sb);
+        wbc->wb_start = jiffies; /* livelock avoidance */
+        spin_lock(&inode_lock);
+        if (!wbc->for_kupdate || list_empty(&wb->b_io))
+                queue_io(wb, wbc->older_than_this);
+
+        while (!list_empty(&wb->b_io)) {
+                struct inode *inode = list_entry(wb->b_io.prev,
+                                                 struct inode, i_list);
+                struct super_block *sb = inode->i_sb;
+                enum sb_pin_state state;
+
+                if (wbc->sb && sb != wbc->sb) {
+                        /* super block given and doesn't
+                           match, skip this inode */
+                        redirty_tail(inode);
+                        continue;
+                }
+                state = pin_sb_for_writeback(wbc, sb);
+
+                if (state == SB_PIN_FAILED) {
+                        requeue_io(inode);
+                        continue;
+                }
+                ret = writeback_sb_inodes(sb, wb, wbc);
 
+                if (state == SB_PINNED)
+                        unpin_sb_for_writeback(sb);
+                if (ret)
+                        break;
+        }
         spin_unlock(&inode_lock);
         /* Leave any unwritten inodes on b_io */
 }
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 76e8903cd204..36520ded3e06 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -34,6 +34,9 @@ struct writeback_control {
         enum writeback_sync_modes sync_mode;
         unsigned long *older_than_this; /* If !NULL, only write back inodes
                                            older than this */
+        unsigned long wb_start;         /* Time writeback_inodes_wb was
+                                           called. This is needed to avoid
+                                           extra jobs and livelock */
         long nr_to_write;               /* Write this many pages, and decrement
                                            this for each page written */
         long pages_skipped;             /* Pages which were not written */