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?
(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?)
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