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Date: Sat, 10 Apr 2021 19:01:04 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Yu Zhao <yuzhao@...gle.com>
CC: Andrew Morton <akpm@...ux-foundation.org>,
<konrad.wilk@...cle.com>, "Dan Streetman" <ddstreet@...e.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/frontswap: fix frontswap_register_ops() race with
swapon and swapoff
On 2021/4/10 18:42, Yu Zhao wrote:
> On Sat, Apr 10, 2021 at 1:30 AM Miaohe Lin <linmiaohe@...wei.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@...wei.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)
>>>
>>
>>
> .
>
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