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