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From:   SeongJae Park <sj@kernel.org>
To:     haoxin <xhao@linux.alibaba.com>
Cc:     SeongJae Park <sj@kernel.org>, akpm@linux-foundation.org,
        damon@lists.linux.dev, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1] mm/damon: add DAMON_OBJ macro
Date:   Fri, 30 Sep 2022 18:04:01 +0000
Message-Id: <20220930180401.67230-1-sj@kernel.org>
In-Reply-To: <61df199f-5579-933e-3f11-35f204f93bf4@linux.alibaba.com>
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Hi Xin,

On Fri, 30 Sep 2022 11:46:26 +0800 haoxin <xhao@linux.alibaba.com> wrote:

> Hi SJ,
> 
> 在 2022/9/22 上午12:41, SeongJae Park 写道:
> > Hi Xin,
> >
> > On Wed, 21 Sep 2022 11:49:42 +0800 Xin Hao <xhao@linux.alibaba.com> wrote:
> >
> >> In damon/sysfs.c file, we use 'container_of' macro to get
> >> damon_sysfs_xxx struct instances, but i think it has a little
> >> inconvenience, because we have to pass three arguments to
> >> 'container_of', and the codes also look a bit long, so there i add a
> >> 'DAMON_OBJ' macro, you just need to pass one arguments, then you can get
> >> the right damon_sysfs_xxx struct instance.
> > Thank you always for your helps and efforts, but I have some comments below.
> >
> >> Signed-off-by: Xin Hao <xhao@linux.alibaba.com>
> >> ---
> >>   include/linux/damon.h |   7 ++
> >>   mm/damon/sysfs.c      | 230 +++++++++++++++++-------------------------
> >>   2 files changed, 102 insertions(+), 135 deletions(-)
> >>
> >> diff --git a/include/linux/damon.h b/include/linux/damon.h
> >> index e7808a84675f..a3b577677caa 100644
> >> --- a/include/linux/damon.h
> >> +++ b/include/linux/damon.h
> >> @@ -24,6 +24,13 @@ static inline unsigned long damon_rand(unsigned long l, unsigned long r)
> >>   	return l + prandom_u32_max(r - l);
> >>   }
> >>   
> >> +/*
> >> + * Get damon_sysfs_xxx relative struct instance.
> >> + */
> >> +#define DAMON_OBJ(_type) ({						\
> >> +	const typeof(((struct _type *)0)->kobj)*__mptr = (kobj);	\
> >> +	(struct _type *)((char *)__mptr - offsetof(struct _type, kobj)); })
> >> +
> > So, this macro assumes two implicit rules.
> > 1. The caller would have a relevant 'struct kobject *' variable called 'kobj',
> >     and
> > 2. The '_type' would have the field 'kobj'.
> >
> > I think the implicit rules could make some people confused, so would be better
> > to be well documented.  Even though those are well documented, I think it
> > cannot intuitively read by everyone.  Making the name better self-explaining
> > might help, but then the length of the code would be not so different.
> >
> > So IMHO, this change makes the code a little bit shorter but unclear to
> > understand what it does.  And at least to my humble eyes, use of
> > 'container_of()' makes the code a little bit more verbose, but clear to
> > understand.  I have no idea how we can make this code shorter while keeping it
> > still easily understandable, and I think the level of verboseness is acceptable
> > for the readability.  So Nack at the moment, sorry.
> 
> I really feel the need to do that， how about keep it in the sysfs.c 
> file, and change it like this：
> 
> #define DAMON_OBJ(_struct) container_of(kobj, struct _struct, kobj)
> 
> it will feel easy to understand and made sense.

I don't think this is enough, at least for my humble eyes.  My questions for
better self-explaining name and removing the implicit rules are not answered.
After all, I don't think the usage of container_of() is a real problem, though
we might always have rooms for improvement.


Thanks,
SJ

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