linux-kernel - Re: [PATCH v2 0/7] mm/hotplug: Only use subsection map in VMEMMAP case

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Message-ID: <20200226034236.GD24216@MiWiFi-R3L-srv>
Date:   Wed, 26 Feb 2020 11:42:36 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>,
        David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, richardw.yang@...ux.intel.com,
        osalvador@...e.de, dan.j.williams@...el.com, rppt@...ux.ibm.com,
        robin.murphy@....com
Subject: Re: [PATCH v2 0/7] mm/hotplug: Only use subsection map in VMEMMAP
 case

On 02/25/20 at 11:02am, Michal Hocko wrote:
> On Tue 25-02-20 10:10:45, David Hildenbrand wrote:
> > >>>  include/linux/mmzone.h |   2 +
> > >>>  mm/sparse.c            | 178 +++++++++++++++++++++++++++++------------
> > >>>  2 files changed, 127 insertions(+), 53 deletions(-)
> > >>
> > >> Why do we need to add so much code to remove a functionality from one
> > >> memory model?
> > > 
> > > Hmm, Dan also asked this before.
> > > 
> > > The adding mainly happens in patch 2, 3, 4, including the two newly
> > > added function defitions, the code comments above them, and those added
> > > dummy functions for !VMEMMAP.
> > 
> > AFAIKS, it's mostly a bunch of newly added comments on top of functions.
> > E.g., the comment for fill_subsection_map() alone spans 12 LOC in total.
> > I do wonder if we have to be that verbose. We are barely that verbose on
> > MM code (and usually I don't see much benefit unless it's a function
> > with many users from many different places).
> 
> I would tend to agree here. Not that I am against kernel doc
> documentation but these are internal functions and the comment doesn't
> really give any better insight IMHO. I would be much more inclined if
> this was the general pattern in the respective file but it just stands
> out.

I saw there are internal functions which have code comments, e.g
shrink_slab() in mm/vmscan.c, not only this one place, there are several
places. I personally prefer to see code comment for function if
possible, this can save time, e.g people can skip the bitmap operation
when read code if not necessary. And here I mainly want to tell there
are different returned value to note different behaviour when call them.

Anyway, it's fine to me to remove them. The two functions are internal,
and not so complicated. I will remove them since you both object.
However, I disagree with the saying that we should not add code comment
for internal function.

Thanks
Baoquan