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

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Message-ID: <20190211190646.j6pdxqirc56inbbe@ca-dmjordan1.us.oracle.com>
Date:   Mon, 11 Feb 2019 14:06:46 -0500
From:   Daniel Jordan <daniel.m.jordan@...cle.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>,
        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>,
        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 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.

> +	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