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

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Message-ID: <20200226091421.GE3771@dhcp22.suse.cz>
Date:   Wed, 26 Feb 2020 10:14:21 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>, 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 Wed 26-02-20 11:42:36, Baoquan He wrote:
> 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.

... yet nobody really cares about different return code. It is an error
that is propagated up the call chain and that's all.

I also like when there is a kernel doc comment that helps to understand
the intented usage, context the function can be called from, potential
side effects, locking requirements and other details people need to know
when calling functions. But have a look at 
/**
 * clear_subsection_map - Clear subsection map of one memory region
 *
 * @pfn - start pfn of the memory range
 * @nr_pages - number of pfns to add in the region
 *
 * This is only intended for hotplug, and clear the related subsection
 * map inside one section.
 *
 * Return:
 * * -EINVAL	- Section already deactived.
 * * 0		- Subsection map is emptied.
 * * 1		- Subsection map is not empty.
 */

the only useful information in here is that this is a hotplug stuff but
I would be completely lost about the intention without already knowing
what is this whole subsection about.

-- 
Michal Hocko
SUSE Labs