Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
IronPort-SDR: LERVPcaK2soudAsveVRK6jvRU8TJQou0PTlZgPapWBxOiyLeLGLasNpHQqIYkTM66/pdzojAyG
 6IddldTcvYmg==
IronPort-SDR: 6qaw/Hnypxc/zSmBohiorkumC/YHiBQk2uyQGEdIfPxIC07OtwwSgEOdFBp03aWqds7HTPISdY
 6caG3+NPKXrQ==
Date:   Tue, 7 Jul 2020 10:38:29 +0800
From:   Feng Tang <feng.tang@intel.com>
To:     Andi Kleen <andi.kleen@intel.com>
Cc:     Qian Cai <cai@lca.pw>, Andrew Morton <akpm@linux-foundation.org>,
        Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        kernel test robot <rong.a.chen@intel.com>,
        Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Kees Cook <keescook@chromium.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Iurii Zaikin <yzaikin@google.com>, tim.c.chen@intel.com,
        dave.hansen@intel.com, ying.huang@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, lkp@lists.01.org
Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail
Message-ID: <20200707023829.GA85993@shbuild999.sh.intel.com>
References: <20200705044454.GA90533@shbuild999.sh.intel.com>
 <FAAE2B23-2565-4F36-B278-018A5AD219EE@lca.pw>
 <20200705125854.GA66252@shbuild999.sh.intel.com>
 <20200705155232.GA608@lca.pw>
 <20200706014313.GB66252@shbuild999.sh.intel.com>
 <20200706023614.GA1231@lca.pw>
 <20200706132443.GA34488@shbuild999.sh.intel.com>
 <20200706133434.GA3483883@tassilo.jf.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200706133434.GA3483883@tassilo.jf.intel.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Jul 06, 2020 at 06:34:34AM -0700, Andi Kleen wrote:
> >  	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> > -	if (ret == 0 && write)
> > +	if (ret == 0 && write) {
> > +		if (sysctl_overcommit_memory == OVERCOMMIT_NEVER)
> > +			schedule_on_each_cpu(sync_overcommit_as);
> 
> The schedule_on_each_cpu is not atomic, so the problem could still happen
> in that window.
> 
> I think it may be ok if it eventually resolves, but certainly needs
> a comment explaining it. Can you do some stress testing toggling the
> policy all the time on different CPUs and running the test on
> other CPUs and see if the test fails?

For the raw test case reported by 0day, this patch passed in 200 times
run. And I will read the ltp code and try stress testing it as you
suggested.


> The other alternative would be to define some intermediate state
> for the sysctl variable and only switch to never once the schedule_on_each_cpu
> returned. But that's more complexity.

One thought I had is to put this schedule_on_each_cpu() before
the proc_dointvec_minmax() to do the sync before sysctl_overcommit_memory
is really changed. But the window still exists, as the batch is
still the larger one. 

Thanks,
Feng

> 
> 
> -Andi