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Subject: Re: [PATCH] mm/frontswap: fix frontswap_register_ops() race with
 swapon and swapoff
To:     Yu Zhao <yuzhao@google.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        <konrad.wilk@oracle.com>, "Dan Streetman" <ddstreet@ieee.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
References: <20210405102008.25504-1-linmiaohe@huawei.com>
 <5ec0f24e-eac2-1e62-628b-757f8ce11dc2@huawei.com>
 <CAOUHufYYjRgO0+3zD6OMn2m3u5qm6sFg5pWxNujnPMaK0hv4Qw@mail.gmail.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <7e5ea991-f219-5b42-ee71-4ec4004ca550@huawei.com>
Date:   Sat, 10 Apr 2021 19:01:04 +0800
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On 2021/4/10 18:42, Yu Zhao wrote:
> On Sat, Apr 10, 2021 at 1:30 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>>
>> Hi:
>> On 2021/4/5 18:20, Miaohe Lin wrote:
>>> frontswap_register_ops can race with swapon. Consider the following scene:
>>
>> Any comment or suggestion? Or is this race window too theoretical to fix?
>> Thanks.
> 
> Let me run a stress test and get back to you (within a day or so).

That's very kind of you. Many thanks!

> 
>>>
>>> CPU1                                  CPU2
>>> ----                                  ----
>>> frontswap_register_ops
>>>   fill bitmap a
>>>   ops->init
>>>                                       sys_swapon
>>>                                         enable_swap_info
>>>                                           frontswap_init without new ops
>>>   add ops to frontswap_ops list
>>>   check if swap_active_head changed
>>>                                           add to swap_active_head
>>>
>>> So the frontswap_ops init is missed on the new swap device. Consider the
>>> another scene:
>>> CPU1                                    CPU2
>>> ----                                    ----
>>> frontswap_register_ops
>>>   fill bitmap a
>>>   ops->init
>>>   add ops to frontswap_ops list
>>>                                         sys_swapon
>>>                                           enable_swap_info
>>>                                             frontswap_init with new ops
>>>                                             add to swap_active_head
>>>   check if swap_active_head changed
>>>   ops->init for new swap device [twice!]
>>>
>>> The frontswap_ops init will be called two times on the new swap device this
>>> time. frontswap_register_ops can also race with swapoff. Consider the
>>> following scene:
>>>
>>> CPU1                                    CPU2
>>> ----                                    ----
>>>                                         sys_swapoff
>>>                                         removed from swap_active_head
>>> frontswap_register_ops
>>>   fill bitmap a
>>>   ops->init without swap device
>>>   add ops to frontswap_ops list
>>>                                             invalidate_area with new ops
>>>   check if swap_active_head changed
>>>
>>> We could call invalidate_area on a swap device under swapoff with frontswap
>>> is uninitialized yet. Fix all these by using swapon_mutex to guard against
>>> race with swapon and add swap_info_get_if_under_swapoff() to collect swap
>>> devices under swapoff.
>>>
>>> Fixes: d1dc6f1bcf1e ("frontswap: allow multiple backends")
>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>> ---
>>>  include/linux/swapfile.h |  2 ++
>>>  mm/frontswap.c           | 40 +++++++++++++++++-----------------------
>>>  mm/swapfile.c            | 13 ++++++++++++-
>>>  3 files changed, 31 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
>>> index e06febf62978..7ae15d917828 100644
>>> --- a/include/linux/swapfile.h
>>> +++ b/include/linux/swapfile.h
>>> @@ -9,8 +9,10 @@
>>>  extern spinlock_t swap_lock;
>>>  extern struct plist_head swap_active_head;
>>>  extern struct swap_info_struct *swap_info[];
>>> +extern struct mutex swapon_mutex;
>>>  extern int try_to_unuse(unsigned int, bool, unsigned long);
>>>  extern unsigned long generic_max_swapfile_size(void);
>>>  extern unsigned long max_swapfile_size(void);
>>> +extern struct swap_info_struct *swap_info_get_if_under_swapoff(int type);
>>>
>>>  #endif /* _LINUX_SWAPFILE_H */
>>> diff --git a/mm/frontswap.c b/mm/frontswap.c
>>> index 130e301c5ac0..c16bfc7550b5 100644
>>> --- a/mm/frontswap.c
>>> +++ b/mm/frontswap.c
>>> @@ -123,12 +123,26 @@ void frontswap_register_ops(struct frontswap_ops *ops)
>>>
>>>       bitmap_zero(a, MAX_SWAPFILES);
>>>       bitmap_zero(b, MAX_SWAPFILES);
>>> -
>>> +     mutex_lock(&swapon_mutex);
>>>       spin_lock(&swap_lock);
>>>       plist_for_each_entry(si, &swap_active_head, list) {
>>>               if (!WARN_ON(!si->frontswap_map))
>>>                       set_bit(si->type, a);
>>>       }
>>> +     /*
>>> +      * There might be some swap devices under swapoff, i.e. they are
>>> +      * removed from swap_active_head but frontswap_invalidate_area()
>>> +      * is not called yet due to swapon_mutex is held here. We must
>>> +      * collect these swap devices and call ops->init on them or they
>>> +      * might invalidate frontswap area while frontswap is uninitialized.
>>> +      */
>>> +     for_each_clear_bit(i, a, MAX_SWAPFILES) {
>>> +             si = swap_info_get_if_under_swapoff(i);
>>> +             if (!si || !si->frontswap_map)
>>> +                     continue;
>>> +             set_bit(si->type, b);
>>> +     }
>>> +     bitmap_or(a, a, b, MAX_SWAPFILES);
>>>       spin_unlock(&swap_lock);
>>>
>>>       /* the new ops needs to know the currently active swap devices */
>>> @@ -144,29 +158,9 @@ void frontswap_register_ops(struct frontswap_ops *ops)
>>>               ops->next = frontswap_ops;
>>>       } while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
>>>
>>> -     static_branch_inc(&frontswap_enabled_key);
>>> -
>>> -     spin_lock(&swap_lock);
>>> -     plist_for_each_entry(si, &swap_active_head, list) {
>>> -             if (si->frontswap_map)
>>> -                     set_bit(si->type, b);
>>> -     }
>>> -     spin_unlock(&swap_lock);
>>> +     mutex_unlock(&swapon_mutex);
>>>
>>> -     /*
>>> -      * On the very unlikely chance that a swap device was added or
>>> -      * removed between setting the "a" list bits and the ops init
>>> -      * calls, we re-check and do init or invalidate for any changed
>>> -      * bits.
>>> -      */
>>> -     if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
>>> -             for (i = 0; i < MAX_SWAPFILES; i++) {
>>> -                     if (!test_bit(i, a) && test_bit(i, b))
>>> -                             ops->init(i);
>>> -                     else if (test_bit(i, a) && !test_bit(i, b))
>>> -                             ops->invalidate_area(i);
>>> -             }
>>> -     }
>>> +     static_branch_inc(&frontswap_enabled_key);
>>>  }
>>>  EXPORT_SYMBOL(frontswap_register_ops);
>>>
>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>> index 149e77454e3c..ee736533717f 100644
>>> --- a/mm/swapfile.c
>>> +++ b/mm/swapfile.c
>>> @@ -89,7 +89,7 @@ static DEFINE_SPINLOCK(swap_avail_lock);
>>>
>>>  struct swap_info_struct *swap_info[MAX_SWAPFILES];
>>>
>>> -static DEFINE_MUTEX(swapon_mutex);
>>> +DEFINE_MUTEX(swapon_mutex);
>>>
>>>  static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
>>>  /* Activity counter to indicate that a swapon or swapoff has occurred */
>>> @@ -2958,6 +2958,17 @@ __weak unsigned long max_swapfile_size(void)
>>>       return generic_max_swapfile_size();
>>>  }
>>>
>>> +struct swap_info_struct *swap_info_get_if_under_swapoff(int type)
>>> +{
>>> +     struct swap_info_struct *si = swap_type_to_swap_info(type);
>>> +
>>> +     if (!si || !si->swap_map)
>>> +             return NULL;
>>> +     if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
>>> +             return si;
>>> +     return NULL;
>>> +}
>>> +
>>>  static unsigned long read_swap_header(struct swap_info_struct *p,
>>>                                       union swap_header *swap_header,
>>>                                       struct inode *inode)
>>>
>>
>>
> .
>