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

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Date:   Tue, 12 Feb 2019 14:40:48 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.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>,
        Jérôme 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>,
        "Andrea Parri" <andrea.parri@...rulasolutions.com>
Subject: Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations

Daniel Jordan <daniel.m.jordan@...cle.com> writes:

> On Mon, Feb 11, 2019 at 04:38:46PM +0800, Huang, Ying wrote:
>> +struct swap_info_struct *get_swap_device(swp_entry_t entry)
>> +{
>> +	struct swap_info_struct *si;
>> +	unsigned long type, offset;
>> +
>> +	if (!entry.val)
>> +		goto out;
>
>> +	type = swp_type(entry);
>> +	si = swap_type_to_swap_info(type);
>
> These lines can be collapsed into swp_swap_info if you want.

Yes.  I can use that function to reduce another line from the patch.
Thanks!  Will do that.

>> +	if (!si)
>> +		goto bad_nofile;
>> +
>> +	preempt_disable();
>> +	if (!(si->flags & SWP_VALID))
>> +		goto unlock_out;
>
> After Hugh alluded to barriers, it seems the read of SWP_VALID could be
> reordered with the write in preempt_disable at runtime.  Without smp_mb()
> between the two, couldn't this happen, however unlikely a race it is?
>
> CPU0                                CPU1
>
> __swap_duplicate()
>     get_swap_device()
>         // sees SWP_VALID set
>                                    swapoff
>                                        p->flags &= ~SWP_VALID;
>                                        spin_unlock(&p->lock); // pair w/ smp_mb
>                                        ...
>                                        stop_machine(...)
>                                        p->swap_map = NULL;
>         preempt_disable()
>     read NULL p->swap_map

Andrea has helped to explain this.

Best Regards,
Huang, Ying