On Mon 09-10-17 15:42:12, Andrew Morton wrote:
> On Sat, 7 Oct 2017 06:58:04 +0800 Yafang Shao <laoar.shao@xxxxxxxxx> 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@xxxxxxxx>
SUSE Labs, CR
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