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Date: Fri, 8 Dec 2017 14:09:09 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: "Huang\, Ying" <ying.huang@intel.com>
Cc: Minchan Kim <minchan@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, <linux-mm@kvack.org>,
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        David Rientjes <rientjes@google.com>, Rik van Riel <riel@redhat.com>,
        Jan Kara <jack@suse.cz>, Dave Jiang <dave.jiang@intel.com>,
        Aaron Lu <aaron.lu@intel.com>
Subject: Re: [PATCH -mm] mm, swap: Fix race between swapoff and some swap
 operations
Message-Id: <20171208140909.4e31ba4f1235b638ae68fd5c@linux-foundation.org>
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On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@intel.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?