Date:   Thu, 26 Mar 2020 19:22:09 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     David Rientjes <rientjes@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        iamjoonsoo.kim@....com, hannes@...xchg.org, vbabka@...e.cz,
        mgorman@...hsingularity.net
Subject: Re: [PATCH 4/5] mm/vmstat.c: move the per-node stats to the front of
 /proc/zoneinfo

On 03/26/20 at 07:43am, Michal Hocko wrote:
> On Thu 26-03-20 12:24:54, Baoquan He wrote:
> > On 03/25/20 at 12:45pm, David Rientjes wrote:
> > > On Wed, 25 Mar 2020, Baoquan He wrote:
> > > 
> > > > > Even this can break existing parsers. Fixing that up is likely not hard
> > > > > and existing parsers would be mostly debugging hacks here and there but
> > > > > I do miss any actual justification except for you considering it more
> > > > > sensible. I do not remember this would be a common pain point for people
> > > > > parsing this file. If anything the overal structure of the file makes it
> > > > > hard to parse and your patches do not really address that. We are likely
> > > > > too late to make the output much more sensible TBH.
> > > > > 
> > > > > That being said, I haven't looked more closely on your patches because I
> > > > > do not have spare cycles for that. Your justification for touching the
> > > > > code which seems to be working relatively well is quite weak IMHO, yet
> > > > > it adds a non zero risk for breaking existing parsers.
> > > > 
> > > > I would take the saying of non zero risk for breaking existing parsers.
> > > > When considering this change, I thought about the possible risk. However,
> > > > found out the per-node stats was added in 2016 which is not so late, and
> > > > assume nobody will rely on the order of per-node stats embeded into a
> > > > zone. But I have to admit any concern or worry of risk is worth being
> > > > considerred carefully since /proc/zoneinfo is a classic interface.
> > > > 
> > > 
> > > For context, we started parsing /proc/zoneinfo in initscripts to be able 
> > > to determine the order in which vm.lowmem_reserve_ratio needs to be set 
> > > and this required my kernel change from 2017:
> > > 
> > > commit b2bd8598195f1b2a72130592125ac6b4218988a2
> > > Author: David Rientjes <rientjes@...gle.com>
> > > Date:   Wed May 3 14:52:59 2017 -0700
> > > 
> > >     mm, vmstat: print non-populated zones in zoneinfo
> > > 
> > > Otherwise, we found, it's much more difficult to determine how this array 
> > > should be structured.  So at least we parse this file very carefully, I'm 
> > > not sure how much others do, but it seems like an unnecessary risk for 
> > > little reward.  I'm happy to see it has been decided to drop this patch 
> > > and patch 5.
> > 
> > 
> > OK, I see why it is in such a situation, the empty zones were not printed. 
> > 
> > I could still not get how vm.lowmem_reserve_ratio is set with
> > /proc/zoneinfo in the old initscripts, do you see any risk if not
> > filling and showing the ->lowmem_reserve[] of empty zone in
> > patch 2 and 3?  Thanks in advance.
> 
> The point is why should we even care. Displaying that information
> shouldn't hurt anything, right?

Well, I would say why not. If saying anything hurted, I often check
/proc/zoneinfo to get information about system memory like many people,
I was wondering why the protection data is over there, but it's am
empty zone, and they protect what. I dare say it's more than once I
asked to myself, just sometime I am too lazy to start to make it clear
when focusing on another issue. Not sure if that is kind of hurting. I
would like to see it's not there to confuse me if anyone else have
stood up to fix it, I absolutely will vote for it.

Surely, we also need to evaluate if any risk or complexity is involved, 
While with my understanding, I don't see risk, and the change is quite
simple and easy to understand.

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