writeback: make the super_block pinning more efficient

Currently we pin the inode->i_sb for every single inode. This
increases cache traffic on sb->s_umount sem. Lets instead
cache the inode sb pin state and keep the super_block pinned
for as long as keep writing out inodes from the same
super_block.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 29 insertions, 17 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 225c7316344e..c6bf775e641a 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -540,6 +540,17 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
         return ret;
 }
 
+static void unpin_sb_for_writeback(struct super_block **psb)
+{
+        struct super_block *sb = *psb;
+
+        if (sb) {
+                up_read(&sb->s_umount);
+                put_super(sb);
+                *psb = NULL;
+        }
+}
+
 /*
  * 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
@@ -549,11 +560,20 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
  * 1 if we failed.
  */
 static int pin_sb_for_writeback(struct writeback_control *wbc,
-                                   struct inode *inode)
+                                struct inode *inode, struct super_block **psb)
 {
         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) {
@@ -566,7 +586,7 @@ static int pin_sb_for_writeback(struct writeback_control *wbc,
         if (down_read_trylock(&sb->s_umount)) {
                 if (sb->s_root) {
                         spin_unlock(&sb_lock);
-                        return 0;
+                        goto pinned;
                 }
                 /*
                  * umounted, drop rwsem again and fall through to failure
@@ -577,24 +597,15 @@ static int pin_sb_for_writeback(struct writeback_control *wbc,
         sb->s_count--;
         spin_unlock(&sb_lock);
         return 1;
-}
-
-static void unpin_sb_for_writeback(struct writeback_control *wbc,
-                                   struct inode *inode)
-{
-        struct super_block *sb = inode->i_sb;
-
-        if (wbc->sync_mode == WB_SYNC_ALL)
-                return;
-
-        up_read(&sb->s_umount);
-        put_super(sb);
+pinned:
+        *psb = sb;
+        return 0;
 }
 
 static void writeback_inodes_wb(struct bdi_writeback *wb,
                                 struct writeback_control *wbc)
 {
-        struct super_block *sb = wbc->sb;
+        struct super_block *sb = wbc->sb, *pin_sb = NULL;
         const int is_blkdev_sb = sb_is_blkdev_sb(sb);
         const unsigned long start = jiffies;    /* livelock avoidance */
 
@@ -653,7 +664,7 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
                 if (inode_dirtied_after(inode, start))
                         break;
 
-                if (pin_sb_for_writeback(wbc, inode)) {
+                if (pin_sb_for_writeback(wbc, inode, &pin_sb)) {
                         requeue_io(inode);
                         continue;
                 }
@@ -662,7 +673,6 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
                 __iget(inode);
                 pages_skipped = wbc->pages_skipped;
                 writeback_single_inode(inode, wbc);
-                unpin_sb_for_writeback(wbc, inode);
                 if (wbc->pages_skipped != pages_skipped) {
                         /*
                          * writeback is not making progress due to locked
@@ -682,6 +692,8 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
                         wbc->more_io = 1;
         }
 
+        unpin_sb_for_writeback(&pin_sb);
+
         spin_unlock(&inode_lock);
         /* Leave any unwritten inodes on b_io */
 }