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

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Date:	Mon, 13 Jan 2014 11:08:58 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Weijie Yang <weijie.yang@...sung.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Shaohua Li <shli@...ionio.com>, Bob Liu <bob.liu@...cle.com>,
	stable@...r.kernel.org,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>
Subject: Re: [PATCH] mm/swap: fix race on swap_info reuse between swapoff and swapon

On Sat, Jan 11, 2014 at 9:11 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Thu, 09 Jan 2014 13:39:55 +0800 Weijie Yang <weijie.yang@...sung.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 :(
>

Hi, Andrew. Thanks for your suggestion.

I checked Krzysztof's patch, it use the global swapon_mutex to protect
race condition among
swapon, swapoff and swap_start(). It is a kind of correct method, but
a heavy method.

I will try to resend a patchset to make lock usage in swapfile.c clear
and fine grit
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