linux-kernel - Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations

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Date:   Thu, 14 Feb 2019 16:47:41 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jï¿½rï¿½me Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Rik van Riel <riel@...hat.com>, Jan Kara <jack@...e.cz>,
        Dave Jiang <dave.jiang@...el.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Subject: Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap
 operations

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.

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.