Re: [PATCH] mm/frontswap: fix frontswap_register_ops() race with swapon and swapoff

From: Miaohe Lin
Date: Sun Apr 11 2021 - 23:08:38 EST

On 2021/4/11 4:02, Yu Zhao wrote:
> On Sat, Apr 10, 2021 at 5:01 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> On 2021/4/10 18:42, Yu Zhao wrote:
>>> On Sat, Apr 10, 2021 at 1:30 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> 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!
>
> I'm still getting the following crash. Probably I should try the other
> series you sent earlier too?
>
> BUG: unable to handle page fault for address: ffffaa7937d82000
> RIP: 0010:scan_swap_map_slots+0x12b/0x7f0
> Call Trace:
> get_swap_pages+0x278/0x590
> get_swap_page+0x1ab/0x280
> add_to_swap+0x7d/0x130
> shrink_page_list+0xf84/0x25f0
> reclaim_pages+0x313/0x430
> madvise_cold_or_pageout_pte_range+0x95c/0xaa0
> walk_p4d_range+0x43f/0x790
> walk_pgd_range+0xf1/0x150
> __walk_page_range+0x6f/0x1b0
> walk_page_range+0xbe/0x1e
> do_madvise+0x271/0x720

Sorry about it! This patch is fix the frontswap_register_ops() race with swapon/swapoff.
And the earlier patch is fix the race I found when I was investigating the swap code. So
this crash may not be included.

>
>>>>> 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@xxxxxxxxxx>
>>>>> ---
>>>>> 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)
>>>>>
>>>>
>>>>
>>> .
>>>
>>
>>
> .
>