Re: [PATCH v2] mm: introduce validity check on vm dirtiness settings

[Yafang Shao]

2017-09-20 23:33 GMT+08:00 Jan Kara <jack@xxxxxxx>:
> On Tue 19-09-17 19:48:00, Yafang Shao wrote:
>> 2017-09-19 16:35 GMT+08:00 Jan Kara <jack@xxxxxxx>:
>> > On Tue 19-09-17 06:53:00, Yafang Shao wrote:
>> >> +     if (vm_dirty_bytes == 0 && vm_dirty_ratio == 0 &&
>> >> +             (dirty_background_bytes != 0 || dirty_background_ratio != 0))
>> >> +             ret = false;
>> >
>> > Hum, why not just:
>> >         if ((vm_dirty_bytes == 0 && vm_dirty_ratio == 0) ||
>> >             (dirty_background_bytes == 0 && dirty_background_ratio == 0))
>> >                 ret = false;
>> >
>> > IMHO setting either tunable to 0 is just wrong and actively dangerous...
>> >
>>
>> Because these four variables all could be set to 0 before, and I'm not
>> sure if this
>> is needed under some certain conditions, although I think this is
>> dangerous but I have
>> to keep it as before.
>>
>> If you think that is wrong, then I will modified it as you suggested.
>
> OK, I see but see below.
>
>> >>  int dirty_background_ratio_handler(struct ctl_table *table, int write,
>> >>               void __user *buffer, size_t *lenp,
>> >>               loff_t *ppos)
>> >>  {
>> >>       int ret;
>> >> +     int old_ratio = dirty_background_ratio;
>> >>
>> >>       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>> >> -     if (ret == 0 && write)
>> >> -             dirty_background_bytes = 0;
>> >> +     if (ret == 0 && write) {
>> >> +             if (dirty_background_ratio != old_ratio &&
>> >> +                     !vm_dirty_settings_valid()) {
>> >
>> > Why do you check whether new ratio is different here? If it is really
>> > needed, it would deserve a comment.
>> >
>>
>> There're two reseaons,
>> 1.  if you set a value same with the old value, it's needn't to do this check.
>> 2. there's another behavior that I'm not sure whether it is reaonable.  i.e.
>>      if the old value is,
>>             vm.dirty_background_bytes = 0;
>>             vm.dirty_background_ratio=10;
>>       then I execute the bellow command,
>>             sysctl -w vm.dirty_background_bytes=0
>>      at the end these two values will be,
>>             vm.dirty_background_bytes = 0;
>>             vm.dirty_background_ratio=0;
>> I'm not sure if this is needed under some certain conditons, So I have
>> to keep it as before.
>
> OK, this is somewhat the problem of the switching logic between _bytes and
> _ratio bytes and also the fact that '0' has a special meaning in these
> files. I think the cleanest would be to just refuse writing of '0' into any
> of these files which would deal with the problem as well.
Got it.
I will submit a new patch then.

>
>                                                                 Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR