Re: [f2fs-dev] [PATCH] f2fs: avoid race condition for shinker count

[Chao Yu <yuchao0@huawei.com>]

On 2020/11/10 12:19, Jaegeuk Kim wrote:
> On 11/10, Chao Yu wrote:
>> On 2020/11/10 1:00, Jaegeuk Kim wrote:
>>> Light reported sometimes shinker gets nat_cnt < dirty_nat_cnt resulting in
>>
>> I didn't get the problem clearly, did you mean __count_nat_entries() will
>> give the wrong shrink count due to race condition? should there be a lock
>> while reading these two variables?
>>
>>> wrong do_shinker work. Basically the two counts should not happen like that.
>>>
>>> So, I suspect this race condtion where:
>>> - f2fs_try_to_free_nats            __flush_nat_entry_set
>>>    nat_cnt=2, dirty_nat_cnt=2
>>>                                      __clear_nat_cache_dirty
>>>                                       spin_lock(nat_list_lock)
>>>                                       list_move()
>>>                                       spin_unlock(nat_list_lock)
>>>    spin_lock(nat_list_lock)
>>>    list_del()
>>>    spin_unlock(nat_list_lock)
>>>    nat_cnt=1, dirty_nat_cnt=2
>>>                                      nat_cnt=1, dirty_nat_cnt=1
>>
>> nm_i->nat_cnt and nm_i->dirty_nat_cnt were protected by
>> nm_i->nat_tree_lock, I didn't see why expanding nat_list_lock range
>> will help... since there are still places nat_list_lock() didn't
>> cover these two reference counts.
> 
> Yeah, I missed nat_tree_lock, and indeed it should cover this. So, the problem
> is Light reported subtle case of nat_cnt < dirty_nat_cnt in shrink_count.
> We may need to use nat_tree_lock in shrink_count?

change like this?

__count_nat_entries()

	down_read(&nm_i->nat_tree_lock);
	count = NM_I(sbi)->nat_cnt - NM_I(sbi)->dirty_nat_cnt;
	up_read(&nm_i->nat_tree_lock);

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>> Reported-by: Light Hsieh <Light.Hsieh@mediatek.com>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>>> ---
>>>    fs/f2fs/node.c | 3 +--
>>>    1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>>> index 42394de6c7eb..e8ec65e40f06 100644
>>> --- a/fs/f2fs/node.c
>>> +++ b/fs/f2fs/node.c
>>> @@ -269,11 +269,10 @@ static void __clear_nat_cache_dirty(struct f2fs_nm_info *nm_i,
>>>    {
>>>    	spin_lock(&nm_i->nat_list_lock);
>>>    	list_move_tail(&ne->list, &nm_i->nat_entries);
>>> -	spin_unlock(&nm_i->nat_list_lock);
>>> -
>>>    	set_nat_flag(ne, IS_DIRTY, false);
>>>    	set->entry_cnt--;
>>>    	nm_i->dirty_nat_cnt--;
>>> +	spin_unlock(&nm_i->nat_list_lock);
>>>    }
>>>    static unsigned int __gang_lookup_nat_set(struct f2fs_nm_info *nm_i,
>>>
> .
>