From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753929AbaIWE40 (ORCPT <rfc822;w@1wt.eu>);
	Tue, 23 Sep 2014 00:56:26 -0400
Received: from lgeamrelo04.lge.com ([156.147.1.127]:55761 "EHLO
	lgeamrelo04.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751595AbaIWE4Z (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 23 Sep 2014 00:56:25 -0400
X-Original-SENDERIP: 10.177.220.156
X-Original-MAILFROM: minchan@kernel.org
Date: Tue, 23 Sep 2014 13:57:01 +0900
From: Minchan Kim <minchan@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Hugh Dickins <hughd@google.com>, Shaohua Li <shli@kernel.org>,
        Jerome Marchand <jmarchan@redhat.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Dan Streetman <ddstreet@ieee.org>, Nitin Gupta <ngupta@vflare.org>,
        Luigi Semenzato <semenzato@google.com>, juno.choi@lge.com
Subject: Re: [PATCH v1 5/5] zram: add fullness knob to control swap full
Message-ID: <20140923045701.GD8325@bbox>
References: <1411344191-2842-1-git-send-email-minchan@kernel.org>
 <1411344191-2842-6-git-send-email-minchan@kernel.org>
 <20140922141733.023a9ecbc0fe802d7f742d6e@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20140922141733.023a9ecbc0fe802d7f742d6e@linux-foundation.org>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Sep 22, 2014 at 02:17:33PM -0700, Andrew Morton wrote:
> On Mon, 22 Sep 2014 09:03:11 +0900 Minchan Kim <minchan@kernel.org> wrote:
> 
> > Some zram usecase could want lower fullness than default 80 to
> > avoid unnecessary swapout-and-fail-recover overhead.
> > 
> > A typical example is that mutliple swap with high piroirty
> > zram-swap and low priority HDD-swap so it could still enough
> > free swap space although one of swap devices is full(ie, zram).
> > It would be better to fail over to HDD-swap rather than failing
> > swap write to zram in this case.
> > 
> > This patch exports fullness to user so user can control it
> > via the knob.
> 
> Adding new userspace interfaces requires a pretty strong justification
> and it's unclear to me that this is being met.  In fact the whole
> patchset reads like "we have some problem, don't know how to fix it so
> let's add a userspace knob to make it someone else's problem".

I explained rationale in 4/5's reply but if it's not enough or wrong,
please tell me.

> 
> > index b13dc993291f..817738d14061 100644
> > --- a/Documentation/ABI/testing/sysfs-block-zram
> > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > @@ -138,3 +138,13 @@ Description:
> >  		amount of memory ZRAM can use to store the compressed data.  The
> >  		limit could be changed in run time and "0" means disable the
> >  		limit.  No limit is the initial state.  Unit: bytes
> > +
> > +What:		/sys/block/zram<id>/fullness
> > +Date:		August 2014
> > +Contact:	Minchan Kim <minchan@kernel.org>
> > +Description:
> > +		The fullness file is read/write and specifies how easily
> > +		zram become full state so if you set it to lower value,
> > +		zram can reach full state easily compared to higher value.
> > +		Curretnly, initial value is 80% but it could be changed.
> > +		Unit: Percentage
> 
> And I don't think that there is sufficient information here for a user
> to be able to work out what to do with this tunable.

I will put more words.

> 
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -136,6 +136,37 @@ static ssize_t max_comp_streams_show(struct device *dev,
> >  	return scnprintf(buf, PAGE_SIZE, "%d\n", val);
> >  }
> >  
> > +static ssize_t fullness_show(struct device *dev,
> > +		struct device_attribute *attr, char *buf)
> > +{
> > +	int val;
> > +	struct zram *zram = dev_to_zram(dev);
> > +
> > +	down_read(&zram->init_lock);
> > +	val = zram->fullness;
> > +	up_read(&zram->init_lock);
> 
> Did we really need to take a lock to display a value which became
> out-of-date as soon as we released that lock?
> 
> > +	return scnprintf(buf, PAGE_SIZE, "%d\n", val);
> > +}
> > +
> > +static ssize_t fullness_store(struct device *dev,
> > +		struct device_attribute *attr, const char *buf, size_t len)
> > +{
> > +	int err;
> > +	unsigned long val;
> > +	struct zram *zram = dev_to_zram(dev);
> > +
> > +	err = kstrtoul(buf, 10, &val);
> > +	if (err || val > 100)
> > +		return -EINVAL;
> 
> This overwrites the kstrtoul() return value.

Will fix.

Thanks for the reivew, Andrew.
-- 
Kind regards,
Minchan Kim