linux-kernel - Re: [PATCH 2/8] writeback: move dirty inodes from super_block to backing_dev

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Date:	Sun, 6 Sep 2009 20:43:59 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com, tytso@....edu,
	akpm@...ux-foundation.org, jack@...e.cz
Subject: Re: [PATCH 2/8] writeback: move dirty inodes from super_block to
	backing_dev_info

On Fri, Sep 04 2009, Christoph Hellwig wrote:
> On Fri, Sep 04, 2009 at 08:53:57AM +0200, Jens Axboe wrote:
> > > +	if (wbc->sync_mode == WB_SYNC_ALL)
> > > +		bdi_wait_on_work_clear(&work);
> > >  }
> > 
> > That doesn't work, you have to wait for on-stack work. So either we just
> > punt and not do anything for WB_SYNC_NONE if the allocation fails, or we
> > punt to stack and do the wait. Since it's a cleaning action and
> > allocation fails, falling back to the stack and waiting seems like the
> > most appropriate choice.
> 
> True, the wait needs to be unconditional.  Updated version below.

(did you forget that patch? it's not there).

> But now that I look at it, I wonder if we should even bother with it.
> bdi_start_writeback is only used in WC_SYNC_NONE mode in
> balance_dirty_pages.  So if we really run so much out of memory that we
> can't allocate the bdi_work we might just throttle and wait for the
> flusher thread to do it's work.  That would get rid of all the
> special cases for the on-stack bdi_work instances.

Dunno, it feels a lot saner to always block there and ensure that we get
the message across.

-- 
Jens Axboe

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