Re: [PATCH] mm/swap: fix race on swap_info reuse between swapoff and swapon

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 09 Jan 2014 13:39:55 +0800 Weijie Yang <weijie.yang@samsung.com> wrote:

> swapoff clear swap_info's SWP_USED flag prematurely and free its resources
> after that. A concurrent swapon will reuse this swap_info while its previous
> resources are not cleared completely.
> 
> These late freed resources are:
> - p->percpu_cluster
> - swap_cgroup_ctrl[type]
> - block_device setting
> - inode->i_flags &= ~S_SWAPFILE
> 
> This patch clear SWP_USED flag after all its resources freed, so that swapon
> can reuse this swap_info by alloc_swap_info() safely.
> 
> ...
>
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1922,7 +1922,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  	p->swap_map = NULL;
>  	cluster_info = p->cluster_info;
>  	p->cluster_info = NULL;
> -	p->flags = 0;
>  	frontswap_map = frontswap_map_get(p);
>  	spin_unlock(&p->lock);
>  	spin_unlock(&swap_lock);
> @@ -1948,6 +1947,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  		mutex_unlock(&inode->i_mutex);
>  	}
>  	filp_close(swap_file, NULL);
> +
> +	/*
> +	* clear SWP_USED flag after all resources freed
> +	* so that swapon can reuse this swap_info in alloc_swap_info() safely
> +	* it is ok to not hold p->lock after we cleared its SWP_WRITEOK
> +	*/
> +	spin_lock(&swap_lock);
> +	p->flags = 0;
> +	spin_unlock(&swap_lock);
> +
>  	err = 0;
>  	atomic_inc(&proc_poll_event);
>  	wake_up_interruptible(&proc_poll_wait);

I didn't look too closely, but this patch might also address the race
which Krzysztof addressed with
http://ozlabs.org/~akpm/mmots/broken-out/swap-fix-setting-page_size-blocksize-during-swapoff-swapon-race.patch.
Can we please check that out?

I do prefer fixing all these swapon-vs-swapoff races with some large,
simple, wide-scope exclusion scheme.  Perhaps SWP_USED is that scheme.

An alternative would be to add another mutex and just make sys_swapon()
and sys_swapoff() 100% exclusive.  But that is plastering yet another
lock over this mess to hide the horrors which lurk within :(

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