linux-kernel - Re: [PATCH 02/13] mm: use page_off

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Date:   Fri, 18 Sep 2020 12:53:58 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Huang Ying <ying.huang@...el.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/13] mm: use page_off_lru()

On Fri, Sep 18, 2020 at 01:09:14PM +0200, Michal Hocko wrote:
> On Fri 18-09-20 04:27:13, Yu Zhao wrote:
> > On Fri, Sep 18, 2020 at 09:37:00AM +0200, Michal Hocko wrote:
> > > On Thu 17-09-20 21:00:40, Yu Zhao wrote:
> > > > This patch replaces the only open-coded __ClearPageActive() with
> > > > page_off_lru(). There is no open-coded __ClearPageUnevictable()s.
> > > > 
> > > > Before this patch, we have:
> > > > 	__ClearPageActive()
> > > > 	add_page_to_lru_list()
> > > > 
> > > > After this patch, we have:
> > > > 	page_off_lru()
> > > > 		if PageUnevictable()
> > > > 			__ClearPageUnevictable()
> > > > 		else if PageActive()
> > > > 			__ClearPageActive()
> > > > 	add_page_to_lru_list()
> > > > 
> > > > Checking PageUnevictable() shouldn't be a problem because these two
> > > > flags are mutually exclusive. Leaking either will trigger bad_page().
> > > 
> > > I am sorry but the changelog is really hard to grasp. What are you
> > > trying to achieve, why and why it is safe. This should be a general
> > > outline for any patch. I have already commented on the previous patch
> > > and asked you for the explanation why removing __ClearPageActive from
> > > this path is desirable and safe. I have specifically asked to clarify
> > > the compound page situation as that is using its oen destructor in the
> > > freeing path and that might result in page_off_lru to be not called.
> > 
> > Haven't I explained we are NOT removing __ClearPageActive()? Is my
> > notion of the code structure above confusing you? Or 'open-coded'
> > could mean different things?
> 
> Please read through my reply carefuly. I am not saying what you are
> doing is wrong. I am expressing a lack of justification which is the
> case throughout this patch series. You do not explain why we need it and
> why reviewers should spend time on this. Because the review is not as
> trivial as looking at the diff.

I appreciate your time. But if you are looking for some grand
justification, I'm afraid I won't be able to give one, because, as it's
titled, this is just a series of cleanup patches.

My questions above are meant to determine which parts are not clear.
Well, I still don't know. So let's try this. What's your impression upon
reading the first sentence of this patch?

> > > > This patch replaces the only open-coded __ClearPageActive() with
> > > > page_off_lru().

Here is how I would think when I read it (which is purely subjective
since I wrote it):

  'replaces the only (an outlier) open-coded (bad) with
   page_off_lru() (the norm)'

It seems to me it has everything I need to know (or say). Of course I
could spell them all out for you if that's how you'd prefer. And if
it's not enough, then please show me some examples and I'll study
them carefully and try my best to follow them.