linux-kernel - Re: [PATCH] fs: Fix busyloop in wb

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Date:	Thu, 17 Sep 2009 11:09:14 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH] fs: Fix busyloop in wb_writeback()

On Wed 16-09-09 20:41:06, Jens Axboe wrote:
> On Wed, Sep 16 2009, Jan Kara wrote:
> > If all inodes are under writeback (e.g. in case when there's only one inode
> > with dirty pages), wb_writeback() with WB_SYNC_NONE work basically degrades
> > to busylooping until I_SYNC flags of the inode is cleared. Fix the problem by
> > waiting on I_SYNC flags of an inode on b_more_io list in case we failed to
> > write anything.
> 
> Interesting, so this will happen if the dirtier and flush thread end up
> "fighting" each other over the same inode. I'll throw this into the
> testing mix.
  Yes, exactly. When you do "dd if=/dev/zero of=/mnt/mydisk" you're quite
likely to observe that.

> How did you notice?
  Well, UML linux doesn't seem to preempt when a process when it is in the
kernel and thus such busyloop simply deadlocks it. BTW another similar
deadlock happens in UML linux in balance_dirty_pages() (process cycles
there waiting for nr_writeback to drop but that never seems to happen). But
I'd blame that to a problem in UML scheduling...

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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