Re: [PATCH 0/7] Per-bdi writeback flusher threads v15

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Sep 01 2009, Jens Axboe wrote:
> On Tue, Sep 01 2009, Jens Axboe wrote:
> > On Tue, Sep 01 2009, Jens Axboe wrote:
> > > On Tue, Sep 01 2009, Theodore Tso wrote:
> > > > On Mon, Aug 31, 2009 at 09:41:26PM +0200, Jens Axboe wrote:
> > > > > 
> > > > > Here's the 15th version of the writeback patches.
> > > > 
> > > > The test-bdi branch of ext4.git tree is a combination of the patches I
> > > > plan to push once the merge window opens, plus v15 per-bdi patches.
> > > > On the ext4 call I've recruited some ext4 developers to do some
> > > > testing and benchmarking *before* the merge window opens.
> > > 
> > > Great, thanks a lot for that!
> > > 
> > > > I've found something potentially wrong with the writeback.  It was
> > > > careful to sync the disk before shutting down the system, and I got
> > > > the following error.   
> > > > 
> > > > [  660.499155] ext4_da_writepages: jbd2_start: 32768 pages, ino 41; err -30
> > > > [  660.505664] Pid: 1848, comm: flush-8:0 Not tainted 2.6.31-rc8-01474-geef6bd0 #362
> > > > [  660.512402] Call Trace:
> > > > [  660.514982]  [<c064412f>] ? printk+0x14/0x16
> > > > [  660.518851]  [<c023ac4a>] ext4_da_writepages+0x213/0x40e
> > > > [  660.523356]  [<c023aa37>] ? ext4_da_writepages+0x0/0x40e
> > > > [  660.528383]  [<c01b65a8>] do_writepages+0x28/0x39
> > > > [  660.533534]  [<c01f1e07>] writeback_single_inode+0x15c/0x346
> > > > [  660.538401]  [<c0165b23>] ? down_read_trylock+0x3e/0x48
> > > > [  660.542921]  [<c01f2892>] generic_sync_wb_inodes+0x2ab/0x37b
> > > > [  660.547805]  [<c01f2a2a>] wb_writeback+0xc8/0xfa
> > > > [  660.551975]  [<c01f2abb>] wb_do_writeback+0x5f/0x118
> > > > [  660.556333]  [<c01f2b95>] bdi_writeback_task+0x21/0x92
> > > > [  660.560838]  [<c01c0f5d>] bdi_start_fn+0x63/0xb2
> > > > [  660.565647]  [<c01c0efa>] ? bdi_start_fn+0x0/0xb2
> > > > [  660.569710]  [<c0162363>] kthread+0x73/0x78
> > > > [  660.573528]  [<c01622f0>] ? kthread+0x0/0x78
> > > > [  660.577355]  [<c011e6f3>] kernel_thread_helper+0x7/0x10
> > > > 
> > > > ... and a very similar failure for inode 354
> > > > 
> > > > [  660.582215] ext4_da_writepages: jbd2_start: 32768 pages, ino 354; err -30
> > > > 
> > > > Inode 41 is /var/lib/urandom/random-seed, and inode 354 is
> > > > /var/run/syslogd.pid.
> > > > 
> > > > The failure occurs if I build w/o the ext4 "next" patches, and the
> > > > failure does not happen if I take build v2.6.31-rc8 without per-bdi
> > > > patches (with or without the ext4 patches).  At a guess it looks like
> > > > the bdi threads are trying to write out the dirty files that were
> > > > written in the shutdown scripts, *after* the file system has been made
> > > > read-only in preparation for unmounting the file system in question.
> > > 
> > > OK, that's definitely something that I will look into today. It isn't
> > > necessarily a writeback bug, but it does rather smell like it.
> > 
> > I can reproduce, looks like it's not getting the sync on ro remount. Fix
> > should be included shortly :-)
> 
> I see what is going on now... From the remount sync path, it's hitting:
> 
>         if (!down_read_trylock(&inode->i_sb->s_umount)) {
>                 requeue_io(inode);
>                 continue;
>         }
> 
> in generic_sync_wb_inodes(), so it's not actually getting writing out
> the dirty inodes for the remount.
> 
> do_remount()
>         down_write(&sb->s_umount);
>         do_remount_sb();
>         sync_filesystem(sb);
>                 __sync_filesystem(sb, 0);
>                 __sync_filesystem(sb, 1);
>                         sync_inodes_sb(sb, 1);
>                                 sync_sb_inodes(wbc);
>                                         generic_sync_sb_inodes();
> 
> Jan, any ideas on how best to get around that?

How about something like this? I wish there was a better solution,
though.

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5430d4e..9703136 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -550,20 +550,11 @@ static void generic_sync_wb_inodes(struct bdi_writeback *wb,
 		 */
 		if (inode_dirtied_after(inode, start))
 			break;
-		/*
-		 * If this fs is currently being u/remounted, leave the
-		 * inode alone
-		 */
-		if (!down_read_trylock(&inode->i_sb->s_umount)) {
-			requeue_io(inode);
-			continue;
-		}
 
 		BUG_ON(inode->i_state & (I_FREEING | I_CLEAR));
 		__iget(inode);
 		pages_skipped = wbc->pages_skipped;
 		writeback_single_inode(inode, wbc);
-		up_read(&inode->i_sb->s_umount);
 		if (wbc->pages_skipped != pages_skipped) {
 			/*
 			 * writeback is not making progress due to locked
@@ -987,7 +978,7 @@ EXPORT_SYMBOL(__mark_inode_dirty);
  * on the writer throttling path, and we get decent balancing between many
  * throttled threads: we don't want them all piling up on inode_sync_wait.
  */
-void generic_sync_sb_inodes(struct writeback_control *wbc)
+static void __generic_sync_sb_inodes(struct writeback_control *wbc)
 {
 	if (wbc->bdi)
 		bdi_start_writeback(wbc);
@@ -1039,6 +1030,25 @@ void generic_sync_sb_inodes(struct writeback_control *wbc)
 	}
 
 }
+
+void generic_sync_sb_inodes(struct writeback_control *wbc)
+{
+	struct super_block *sb = wbc->sb;
+
+	if (sb) {
+		spin_lock(&sb_lock);
+		sb->s_count++;
+		spin_unlock(&sb_lock);
+	}
+
+	__generic_sync_sb_inodes(wbc);
+
+	if (sb) {
+		spin_lock(&sb_lock);
+		__put_super_and_need_restart(sb);
+		spin_unlock(&sb_lock);
+	}
+}
 EXPORT_SYMBOL_GPL(generic_sync_sb_inodes);
 
 static void sync_sb_inodes(struct writeback_control *wbc)

-- 
Jens Axboe