linux-kernel - Re: [PATCH] Fix queueing work if !bdi_cap_writeback

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Message-ID: <20120913003337.GA16363@localhost>
Date:	Thu, 13 Sep 2012 08:33:37 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	viro@...iv.linux.org.uk, jack@...e.cz, hch@....de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix queueing work if !bdi_cap_writeback_dirty()

On Wed, Sep 12, 2012 at 05:00:48PM +0900, OGAWA Hirofumi wrote:
> Fengguang Wu <fengguang.wu@...el.com> writes:
> 
> > On Wed, Sep 12, 2012 at 03:28:42AM +0900, OGAWA Hirofumi wrote:
> >> 
> >> If bdi has BDI_CAP_NO_WRITEBACK, bdi_forker_thread() doesn't start
> >> writeback thread. This means there is no consumer of work item made
> >> by bdi_queue_work().
> >> 
> >> This adds to checking of !bdi_cap_writeback_dirty(sb->s_bdi) before
> >> calling bdi_queue_work(), otherwise queued work never be consumed.
> >
> > Thanks for catching this! Does this bug have any side effects other
> > than memory leaking?
> >
> > It may be possible for some caller that actually expect it to do some
> > work to make progress, otherwise will eventually block.  If so, we'll
> > need to fix the caller.
> 
> If used custom bdi with BDI_CAP_NO_WRITEBACK, wait_for_completion()
> (e.g. sync_inodes_sb()) will be blocked forever.

The sync(2) block cannot be fixed by this patch?

> I tested by custom bdi with BDI_CAP_NO_WRITEBACK - sync(2) blocked
> forever by this reason.

What's your test script? How do you create/use that custom bdi?

Thanks,
Fengguang
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