linux-kernel - Re: [PATCH] mm/page-writeback.c: fix bug caused by disable periodic writeback

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Date:   Tue, 10 Oct 2017 11:33:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
        mhocko@...e.com, Johannes Weiner <hannes@...xchg.org>,
        vdavydov.dev@...il.com, jlayton@...hat.com, nborisov@...e.com,
        Theodore Ts'o <tytso@....edu>, mawilcox@...rosoft.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] mm/page-writeback.c: fix bug caused by disable periodic
 writeback

On Tue 10-10-17 17:14:48, Yafang Shao wrote:
> 2017-10-10 16:48 GMT+08:00 Jan Kara <jack@...e.cz>:
> > On Tue 10-10-17 16:00:29, Yafang Shao wrote:
> >> 2017-10-10 6:42 GMT+08:00 Andrew Morton <akpm@...ux-foundation.org>:
> >> > On Sat,  7 Oct 2017 06:58:04 +0800 Yafang Shao <laoar.shao@...il.com> wrote:
> >> >
> >> >> After disable periodic writeback by writing 0 to
> >> >> dirty_writeback_centisecs, the handler wb_workfn() will not be
> >> >> entered again until the dirty background limit reaches or
> >> >> sync syscall is executed or no enough free memory available or
> >> >> vmscan is triggered.
> >> >> So the periodic writeback can't be enabled by writing a non-zero
> >> >> value to dirty_writeback_centisecs
> >> >> As it can be disabled by sysctl, it should be able to enable by
> >> >> sysctl as well.
> >> >>
> >> >> ...
> >> >>
> >> >> --- a/mm/page-writeback.c
> >> >> +++ b/mm/page-writeback.c
> >> >> @@ -1972,7 +1972,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
> >> >>  int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
> >> >>       void __user *buffer, size_t *length, loff_t *ppos)
> >> >>  {
> >> >> -     proc_dointvec(table, write, buffer, length, ppos);
> >> >> +     unsigned int old_interval = dirty_writeback_interval;
> >> >> +     int ret;
> >> >> +
> >> >> +     ret = proc_dointvec(table, write, buffer, length, ppos);
> >> >> +     if (!ret && !old_interval && dirty_writeback_interval)
> >> >> +             wakeup_flusher_threads(0, WB_REASON_PERIODIC);
> >> >> +
> >> >>       return 0;
> >> >
> >> > We could do with a code comment here, explaining why this code exists.
> >> >
> >>
> >> OK. I will comment here.
> >>
> >> > And...  I'm not sure it works correctly?  For example, if a device
> >> > doesn't presently have bdi_has_dirty_io() then wakeup_flusher_threads()
> >> > will skip it and the periodic writeback still won't be started?
> >> >
> >>
> >> That's an issue.
> >> The periodic writeback won't be started.
> >>
> >> Maybe we'd better call  wb_wakeup_delayed(wb) here to bypass the
> >> bdi_has_dirty_io() check ?
> >
> > Well, wb_wakeup_delayed() would be more appropriate but you'd then have to
> > iterate over all bdis and wbs to be able to call it which IMO isn't worth
> > the pain for a special case like this. But the decision is worth mentioning
> > in the comment. Also wakeup_flusher_threads() does in principle what you
> > need - see my reply to Andrew for details.
> >
> >                                                                 Honza
> 
> Thanks for your explaination. I understood.
> I will mention it in the comment.
> 
> Should we do the wakeup whenever dirty_writeback_interval changes ?
> If we still use wakeup_flusher_threads(), it will wakeup the flusher
> threads immediately after we make the change.

Yes, I think we should wakeup for every change of dirty_writeback_interval.
And immediate wakeup is not a problem IMO.

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