Date: Tue, 26 Sep 2017 13:26:56 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Yafang Shao <laoar.shao@gmail.com>
Cc: Jan Kara <jack@suse.cz>, akpm@linux-foundation.org,
        Johannes Weiner <hannes@cmpxchg.org>, vdavydov.dev@gmail.com,
        jlayton@redhat.com, nborisov@suse.com, "Theodore Ts'o" <tytso@mit.edu>,
        mawilcox@microsoft.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm: introduce validity check on vm dirtiness settings
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On Tue 26-09-17 19:06:37, Yafang Shao wrote:
> 2017-09-26 18:25 GMT+08:00 Michal Hocko <mhocko@kernel.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.

> > 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.
-- 
Michal Hocko
SUSE Labs