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

[Yafang Shao <laoar.shao@gmail.com>]

2017-10-10 16:48 GMT+08:00 Jan Kara <jack@suse.cz>:
> On Tue 10-10-17 16:00:29, Yafang Shao wrote:
>> 2017-10-10 6:42 GMT+08:00 Andrew Morton <akpm@linux-foundation.org>:
>> > On Sat,  7 Oct 2017 06:58:04 +0800 Yafang Shao <laoar.shao@gmail.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.

Thanks
Yafang