linux-kernel - Re: [PATCH 7/7] fs-writeback: only allow one inflight and pending full flush

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Date:   Mon, 25 Sep 2017 08:48:46 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, hannes@...xchg.org, clm@...com
Subject: Re: [PATCH 7/7] fs-writeback: only allow one inflight and pending
 full flush

On 09/25/2017 03:35 AM, Jan Kara wrote:
> On Thu 21-09-17 10:00:25, Jens Axboe wrote:
>> On 09/21/2017 09:36 AM, Jens Axboe wrote:
>>>> But more importantly once we are not guaranteed that we only have
>>>> a single global wb_writeback_work per bdi_writeback we should just
>>>> embedd that into struct bdi_writeback instead of dynamically
>>>> allocating it.
>>>
>>> We could do this as a followup. But right now the logic is that we
>>> can have on started (inflight), and still have one new queued.
>>
>> Something like the below would fit on top to do that. Gets rid of the
>> allocation and embeds the work item for global start-all in the
>> bdi_writeback structure.
> 
> Hum, so when we consider stuff like embedded work item, I would somewhat
> prefer to handle this like we do for for_background and for_kupdate style
> writeback so that we don't have another special case. For these don't queue
> any item, we just queue writeback work into the workqueue (via
> wb_wakeup()). When flusher work gets processed wb_do_writeback() checks
> (after processing all normal writeback requests) whether conditions for
> these special writeback styles are met and if yes, it creates on-stack work
> item and processes it (see wb_check_old_data_flush() and
> wb_check_background_flush()).

Thanks Jan, I think that's a really good suggestion and kills the
special case completely. I'll rework the patch as a small series
for 4.15.

> So in this case we would just set some flag in bdi_writeback when memory
> reclaim needs help and wb_do_writeback() would check for this flag and
> create and process writeback-all style writeback work. Granted this does
> not preserve ordering of requests (basically any specific request gets
> priority over writeback-whole-world request) but memory gets cleaned in
> either case so flusher should be doing what is needed.

I don't think that matters, and we're already mostly there since we
reject a request if one is pending. And at this point they are all
identical "start all writeback" requests.

-- 
Jens Axboe