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 11:35:32 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jens Axboe <axboe@...nel.dk>
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, jack@...e.cz
Subject: Re: [PATCH 7/7] fs-writeback: only allow one inflight and pending
 full flush

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()).

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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR