From: "Huang\, Ying" <ying.huang@intel.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
"Minchan Kim" <minchan@kernel.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Hugh Dickins" <hughd@google.com>,
"Johannes Weiner" <hannes@cmpxchg.org>,
"Tim Chen" <tim.c.chen@linux.intel.com>,
"Shaohua Li" <shli@fb.com>,
"Mel Gorman" <mgorman@techsingularity.net>,
"J�r�me Glisse" <jglisse@redhat.com>,
"Michal Hocko" <mhocko@suse.com>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"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
Date: Tue, 12 Dec 2017 09:12:20 +0800 [thread overview]
Message-ID: <87374grbpn.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <20171211170449.GS7829@linux.vnet.ibm.com> (Paul E. McKenney's message of "Mon, 11 Dec 2017 09:04:49 -0800")
Hi, Pual,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote:
>> Andrew Morton <akpm@linux-foundation.org> writes:
>>
>> > 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?
>>
>> 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
next prev parent reply other threads:[~2017-12-12 1:12 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-07 1:14 [PATCH -mm] mm, swap: Fix race between swapoff and some swap operations Huang, Ying
2017-12-08 0:29 ` Andrew Morton
2017-12-08 1:43 ` Minchan Kim
[not found] ` <87po7pg4jt.fsf@yhuang-dev.intel.com>
2017-12-08 8:26 ` Minchan Kim
2017-12-08 8:41 ` Huang, Ying
2017-12-08 9:10 ` Minchan Kim
2017-12-08 12:32 ` Huang, Ying
2017-12-13 7:15 ` Minchan Kim
2017-12-13 8:52 ` Huang, Ying
2017-12-08 22:09 ` Andrew Morton
2017-12-11 5:30 ` Huang, Ying
2017-12-11 17:04 ` Paul E. McKenney
2017-12-12 1:12 ` Huang, Ying [this message]
2017-12-12 17:11 ` Paul E. McKenney
2017-12-13 2:17 ` Huang, Ying
2017-12-13 3:27 ` Paul E. McKenney
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