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

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Date:   Tue, 10 Oct 2017 10:48:17 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        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 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
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR