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

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Date:	Fri, 14 Sep 2012 20:13:05 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Fengguang Wu <fengguang.wu@...el.com>
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()

OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> writes:

> 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.
>>> 
>>> 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.
>
> I guess nobody call it in current kernel though. Hmm.., but we also have
> check in __mark_inode_dirty(), nobody should be using it, right?
>
> If we defined it as the bug, I can't see what BDI_CAP_NO_WRITEBACK wants
> to do actually.  We are not going to allow to disable the writeback task?

ping.

You are saying we should change bdi_cap_writeback_dirty(bdi) to
VM_BUG_ON() too in __mark_inode_dirty()? And probably, you are going to
remove the usage of BDI_CAP_NO_WRITEBACK for sb->s_bdi?

> I was going to use this to disable writeback task on my developing FS...
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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