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

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Message-ID: <20170926133320.GD13627@quack2.suse.cz>
Date:   Tue, 26 Sep 2017 15:33:20 +0200
From:   Jan Kara <jack@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Yafang Shao <laoar.shao@...il.com>, 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 13:54:23, Michal Hocko wrote:
> On Tue 26-09-17 19:45:45, Yafang Shao wrote:
> > >> > 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.

So I personally think that the checks Yafang added are worth the extra
code. The situation with ratio/bytes interface and hard/background limit is
complex enough that it makes sense to have basic sanity checks to me. That
being said I don't have too strong opinion on this so just documentation
update would be also fine by me.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR