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

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Date:   Tue, 10 Oct 2017 10:45:01 +0200
From:   Jan Kara <jack@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Yafang Shao <laoar.shao@...il.com>, jack@...e.cz, mhocko@...e.com,
        hannes@...xchg.org, vdavydov.dev@...il.com, jlayton@...hat.com,
        nborisov@...e.com, 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 Mon 09-10-17 15:42:12, Andrew Morton wrote:
> 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.
> 
> 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?

This works correctly. For this case __mark_inode_dirty() has:

      if (bdi_cap_writeback_dirty(wb->bdi) && wakeup_bdi)
              wb_wakeup_delayed(wb);

So periodic writeback gets automatically started once first dirty inode
appears on a bdi.

> (why does the dirty_writeback_interval==0 special case exist, btw? 
> Seems to be a strange thing to do).

I guess to prevent busylooping? But I'm not sure...
 
> (and what happens if the interval was set to 1 hour and the user
> rewrites that to 1 second?  Does that change take 1 hour to take
> effect?)

That's a good point I didn't think about. So probably we should do the
wakeup whenever dirty_writeback_interval changes. 

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