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

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Date:   Tue, 12 Dec 2017 09:12:20 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>, <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

Hi, Pual,

"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:

> On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote:
>> Andrew Morton <akpm@...ux-foundation.org> writes:
>> 
>> > 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?
>> 
>> Sorry, I don't know how to solve the problem with stop_machine().
>> 
>> The problem we try to resolved is that, we have a swap entry, but that
>> swap entry can become invalid because of swappoff between we check it
>> and we use it.  So we need to prevent swapoff to be run between checking
>> and using.
>> 
>> I don't know how to use stop_machine() in swapoff to wait for all users
>> of swap entry to finish.  Anyone can help me on this?
>
> You can think of stop_machine() as being sort of like a reader-writer
> lock.  The readers can be any section of code with preemption disabled,
> and the writer is the function passed to stop_machine().
>
> Users running real-time applications on Linux don't tend to like
> stop_machine() much, but perhaps it is nevertheless the right tool
> for this particular job.

Thanks a lot for explanation!  Now I understand this.

Another question, for this specific problem, I think both stop_machine()
based solution and rcu_read_lock/unlock() + synchronize_rcu() based
solution work.  If so, what is the difference between them?  I guess rcu
based solution will be a little better for real-time applications?  So
what is the advantage of stop_machine() based solution?

Best Regards,
Huang, Ying