linux-kernel - Re: Bug in kernel 2.6.31, Slow wb

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Date:	Wed, 29 Jul 2009 18:28:07 -0700
From:	Martin Bligh <mbligh@...gle.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Chad Talbott <ctalbott@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Michael Rubin <mrubin@...gle.com>,
	Andrew Morton <akpm@...gle.com>, sandeen@...hat.com,
	Michael Davidson <md@...gle.com>
Subject: Re: Bug in kernel 2.6.31, Slow wb_kupdate writeout

On Wed, Jul 29, 2009 at 5:19 PM, Martin Bligh<mbligh@...gle.com> wrote:
> BTW, can you explain this code at the bottom of generic_sync_sb_inodes
> for me?
>
>                if (wbc->nr_to_write <= 0) {
>                        wbc->more_io = 1;
>                        break;
>                }
>
> I don't understand why we are setting more_io here? AFAICS, more_io
> means there's more stuff to write ... I would think we'd set this if
> nr_to_write was > 0 ?
>
> Or just have the section below brought up above this
> break check and do:
>
> if (!list_empty(&sb->s_more_io) || !list_empty(&sb->s_io))
>        wbc->more_io = 1;
>
> Am I just misunderstanding the intent of more_io ?

I am thinking along the lines of:

@@ -638,13 +609,11 @@ sync_sb_inodes(struct super_block *sb, s
                iput(inode);
                cond_resched();
                spin_lock(&inode_lock);
-               if (wbc->nr_to_write <= 0) {
-                       wbc->more_io = 1;
+               if (wbc->nr_to_write <= 0)
                        break;
-               }
-               if (!list_empty(&sb->s_more_io))
-                       wbc->more_io = 1;
        }
+       if (!list_empty(&sb->s_more_io) || !list_empty(&sb->s_io)
+               wbc->more_io = 1;
        return;         /* Leave any unwritten inodes on s_io */
 }
--
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