Re: [PATCH 1/3 v2] mm: Stop background writeback if there is other work queued for the thread

[Jens Axboe <jaxboe@fusionio.com>]

On 08/08/2010 03:29 AM, Wu Fengguang wrote:
> On Sun, Aug 08, 2010 at 12:12:30PM +0800, Dave Chinner wrote:
>> On Thu, Aug 05, 2010 at 04:45:35PM -0700, Andrew Morton wrote:
>>> On Thu,  5 Aug 2010 20:53:17 +0200
>>> Jan Kara <jack@suse.cz> wrote:
>>>> ---
>>>>  fs/fs-writeback.c |    8 ++++++++
>>>>  1 files changed, 8 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>>>> index d5be169..542471e 100644
>>>> --- a/fs/fs-writeback.c
>>>> +++ b/fs/fs-writeback.c
>>>> @@ -633,6 +633,14 @@ static long wb_writeback(struct bdi_writeback *wb,
>>>>  			break;
>>>>  
>>>>  		/*
>>>> +		 * Background writeout and kupdate-style writeback are
>>>> +		 * easily livelockable. Stop them if there is other work
>>>> +		 * to do so that e.g. sync can proceed.
>>>> +		 */
>>>> +		if ((work->for_background || work->for_kupdate) &&
>>>> +		    !list_empty(&wb->bdi->work_list))
>>>> +			break;
>>>> +		/*
>>>
>>> So what happens if an application sits in a loop doing write&fsync to a
>>> file?  The vm's call for help gets ignored and your data doesn't get
>>> written back for three days??
>>
>> To avoid the possibility of any such occurrence, perhaps requeuing
>> the work rather than cancelling it would be better? i.e. stop, put
>> it behind whatever work just came in and so when the new work
>> completes, we restart the background/expiry based writeback?
> 
> That would be better. Ie. add a flag BDI_background_writeback to
> indicate the background work should be started after finishing with
> the queued works. bdi_start_background_writeback() can be modified to
> simply set the bit and wake up the flusher thread.

I think that is better, we ideally want background writeout to
interleave smoothly with any other write activity. But why not just
mark the 'background writeout was interrupted' bit here when breaking,
and have the thread check-clear that when finishing some other piece
of work (or when going idle)?

-- 
Jens Axboe


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