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

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Message-ID: <20120913063906.GA24974@localhost>
Date:	Thu, 13 Sep 2012 14:39:06 +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 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.
> 
> Signed-off-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
> ---
> 
>  fs/fs-writeback.c |    7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff -puN fs/fs-writeback.c~noop_backing_dev_info-check-fix fs/fs-writeback.c
> --- linux/fs/fs-writeback.c~noop_backing_dev_info-check-fix	2012-09-11 06:12:30.000000000 +0900
> +++ linux-hirofumi/fs/fs-writeback.c	2012-09-11 06:12:30.000000000 +0900
> @@ -120,6 +120,9 @@ __bdi_start_writeback(struct backing_dev
>  {
>  	struct wb_writeback_work *work;
>  
> +	if (!bdi_cap_writeback_dirty(bdi))
> +		return;

Will someone in the current kernel actually call
__bdi_start_writeback() on a BDI_CAP_NO_WRITEBACK bdi?

If the answer is no, VM_BUG_ON(!bdi_cap_writeback_dirty(bdi)) looks better.

Thanks,
Fengguang
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