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

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Date:   Tue, 26 Sep 2017 13:54:23 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yafang Shao <laoar.shao@...il.com>
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

On Tue 26-09-17 19:45:45, Yafang Shao wrote:
> 2017-09-26 19:26 GMT+08:00 Michal Hocko <mhocko@...nel.org>:
> > On Tue 26-09-17 19:06:37, Yafang Shao wrote:
[...]
> >> 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.

Why do we need to update the code then?

> >> > 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 ?

What kind of examples would you like to see. I meant that if the current
logic of bacground vs. direct limit changes the code to check it which
is at a different place IIRC would have to be kept in sync.

That being said, this is my personal opinion, I will not object if there
is a general consensus on merging this. I just believe that this is not
simply worth adding a single line of code. You can then a lot of harm by
setting different values which would pass the added check.
-- 
Michal Hocko
SUSE Labs