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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, Hugh Dickins <hughd@google.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        =?utf-8?Q?J=EF=BF=BDr=EF=BF=BDme_Glisse?= <jglisse@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Rik van Riel <riel@redhat.com>, Jan Kara <jack@suse.cz>,
        Dave Jiang <dave.jiang@intel.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Andrea Parri <andrea.parri@amarulasolutions.com>
Subject: Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations
References: <20190211083846.18888-1-ying.huang@intel.com>
        <20190214023805.GA19090@redhat.com>
        <87k1i2oks6.fsf@yhuang-dev.intel.com>
        <20190214214741.GB10698@redhat.com>
Date:   Fri, 15 Feb 2019 15:50:21 +0800
In-Reply-To: <20190214214741.GB10698@redhat.com> (Andrea Arcangeli's message
        of "Thu, 14 Feb 2019 16:47:41 -0500")
Message-ID: <87mumxa3sy.fsf@yhuang-dev.intel.com>
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Andrea Arcangeli <aarcange@redhat.com> writes:

> On Thu, Feb 14, 2019 at 04:07:37PM +0800, Huang, Ying wrote:
>> Before, we choose to use stop_machine() to reduce the overhead of hot
>> path (page fault handler) as much as possible.  But now, I found
>> rcu_read_lock_sched() is just a wrapper of preempt_disable().  So maybe
>> we can switch to RCU version now.
>
> rcu_read_lock looks more efficient than rcu_read_lock_sched. So for
> this purpose in the fast path rcu_read_lock()/unlock() should be the
> preferred methods, no need to force preempt_disable() (except for
> debug purposes if sleep debug is enabled). Server builds are done with
> voluntary preempt (no preempt shouldn't even exist as config option)
> and there rcu_read_lock might be just a noop.

If

CONFIG_PREEMPT_VOLUNTARY=y
CONFIG_PREEMPT=n
CONFIG_PREEMPT_COUNT=n

which is common for servers,

rcu_read_lock() will be a noop, rcu_read_lock_sched() and
preempt_disable() will be barrier().  So rcu_read_lock() is a little
better.

> Against a fast path rcu_read_lock/unlock before the consolidation
> synchronize_rcu would have been enough, now after the consolidation
> even more certain that it's enough because it's equivalent to _mult.

Yes.  Will change to rcu_read_lock/unlock based method.

Best Regards,
Huang, Ying