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Date:   Fri, 8 Dec 2017 14:09:09 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Huang\, Ying" <ying.huang@...el.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Johannes Weiner" <hannes@...xchg.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Shaohua Li <shli@...com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        J�r�me Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Rik van Riel <riel@...hat.com>, Jan Kara <jack@...e.cz>,
        Dave Jiang <dave.jiang@...el.com>,
        Aaron Lu <aaron.lu@...el.com>
Subject: Re: [PATCH -mm] mm, swap: Fix race between swapoff and some swap
 operations

On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@...el.com> wrote:

> > Why do we need srcu here? Is it enough with rcu like below?
> >
> > It might have a bug/room to be optimized about performance/naming.
> > I just wanted to show my intention.
> 
> Yes.  rcu should work too.  But if we use rcu, it may need to be called
> several times to make sure the swap device under us doesn't go away, for
> example, when checking si->max in __swp_swapcount() and
> add_swap_count_continuation().  And I found we need rcu to protect swap
> cache radix tree array too.  So I think it may be better to use one
> calling to srcu_read_lock/unlock() instead of multiple callings to
> rcu_read_lock/unlock().

Or use stop_machine() ;)  It's very crude but it sure is simple.  Does
anyone have a swapoff-intensive workload?

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