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

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Date:   Tue, 10 Oct 2017 16:00:29 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     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

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 ?
But then I find another issue exisit in the periodic writeback, in
function wb_workfn().

    } else if (wb_has_dirty_io(wb) && dirty_writeback_interval) {
        wb_wakeup_delayed(wb);
    }

>From the above code, we can find that if wb_has_dirty_io return false,
then bdi_writeback will not be wakeup until some other conditions
happen.
Seems we have to check periodically no matther whether there's dirty
IO or not ?

But then, introduce another issue,
If there's no dirty IO but we wakeup the bdi_writeback periodically or
do some other periodic check, there will be  performance hit .

Per my understanding, maybe the periodic writeback needs reimplement.

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

I agree with you.
we'd better impelment as bellow?
    if (!ret && write && dirty_writeback_interval &&
dirty_writeback_interval != old_interva)
        do_something();

> (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?)
>

If we rewirte it as above.
It will wakeup the bdi_writeback immdiately, see bellow:
    wakeup_flusher_threads
        mod_delayed_work(bdi_wq, &wb->dwork, 0);   <<< here's 0.
Next time, it will run periodically.

But is this a good implementation ?
Should we wakeup the bdi_writeback after the interval that we set?
That means, using  wb_wakeup_delayed() instead of
wakeup_flusher_threads(), that's I prefer to.

Thanks
Yafang