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

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Message-Id: <20171009154212.bdf3645a2dce5d540657914b@linux-foundation.org>
Date:   Mon, 9 Oct 2017 15:42:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     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 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?

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

(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?)