linux-kernel - Re: [PATCH v3] mm: introduce validity check on vm dirtiness settings

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Message-ID: <CALOAHbB-H8vtGH4PE8Tr+jmvrQZc3bRXqnG9R1QBQfJKvaHP4g@mail.gmail.com>
Date:   Tue, 26 Sep 2017 19:45:45 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, akpm@...ux-foundation.org,
        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
Subject: Re: [PATCH v3] mm: introduce validity check on vm dirtiness settings

2017-09-26 19:26 GMT+08:00 Michal Hocko <mhocko@...nel.org>:
> On Tue 26-09-17 19:06:37, Yafang Shao wrote:
>> 2017-09-26 18:25 GMT+08:00 Michal Hocko <mhocko@...nel.org>:
>> > On Wed 20-09-17 06:43:35, Yafang Shao wrote:
>> >> we can find the logic in domain_dirty_limits() that
>> >> when dirty bg_thresh is bigger than dirty thresh,
>> >> bg_thresh will be set as thresh * 1 / 2.
>> >>       if (bg_thresh >= thresh)
>> >>               bg_thresh = thresh / 2;
>> >>
>> >> But actually we can set vm background dirtiness bigger than
>> >> vm dirtiness successfully. This behavior may mislead us.
>> >> We'd better do this validity check at the beginning.
>> >
>> > This is an admin only interface. You can screw setting this up even
>> > when you keep consistency between the background and direct limits. In
>> > general we do not try to be clever for these knobs because we _expect_
>> > admins to do sane things. Why is this any different and why do we need
>> > to add quite some code to handle one particular corner case?
>> >
>>
>> Of course we expect admins to do the sane things, but not all admins
>> are expert or faimilar with linux kernel source code.
>> If we have to read the source code to know what is the right thing to
>> do, I don't think this is a good interface, even for the admin.
>
> Well, it is kind of natural to setup background below the direct limit
> in general so I am not sure what is so surprising here. Moreover setting
> a non default drity limits already requires some expertise. It is not
> like an arbitrary value will work just fine...
>
>> Anyway, there's no document on that direct limits should not less than
>> background limits.
>
> Then improve the documentation.

I have improved the kernel documentation as well, in order to make it
more clear for the newbies.

>> > To be honest I am not entirely sure this is worth the code and the
>> > future maintenance burden.
>> I'm not sure if this code is a burden for the future maintenance, but
>> I think that if we don't introduce this code it is a burden to the
>> admins.
>
> anytime we might need to tweak background vs direct limit we would have
> to change these checks as well and that sounds like a maint. burden to
> me.

Would pls. show me some example ?

> --
> Michal Hocko
> SUSE Labs