linux-kernel - Re: dirty balancing deadlock

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Message-Id: <20070218155916.0d3c73a9.akpm@linux-foundation.org>
Date:	Sun, 18 Feb 2007 15:59:16 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: dirty balancing deadlock

On Mon, 19 Feb 2007 00:22:11 +0100 Miklos Szeredi <miklos@...redi.hu> wrote:

> > If so, writes to B will decrease the dirty memory threshold.
> 
> Yes, but not by enough.  Say A dirties a 1100 pages, limit is 1000.
> Some pages queued for writeback (doesn't matter how much).  B writes
> back 1, 1099 dirty remain in A, zero in B.  balance_dirty_pages() for
> B doesn't know that there's nothing more to write back for B, it's
> just waiting there for those 1099, which'll never get written.

hm, OK, arguable.  I guess something like this..

--- a/fs/fs-writeback.c~a
+++ a/fs/fs-writeback.c
@@ -356,7 +356,7 @@ int generic_sync_sb_inodes(struct super_
 			continue;		/* Skip a congested blockdev */
 		}
 
-		if (wbc->bdi && bdi != wbc->bdi) {
+		if (wbc->bdi && bdi != wbc->bdi && bdi_write_congested(bdi)) {
 			if (!sb_is_blkdev_sb(sb))
 				break;		/* fs has the wrong queue */
 			list_move(&inode->i_list, &sb->s_dirty);
_

but where's pdflush?  It should be busily transferring dirtiness from A to
B.

> > The writeout code _should_ just sit there transferring dirtyiness from A to
> > B and cleaning pages via B, looping around, alternating between both.
> > 
> > What does sysrq-t say?
> 
> This is the fuse daemon thread that got stuck.

Where's pdflsuh?
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