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Subject: Re: [PATCH v4] f2fs: add bio cache for IPU
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-f2fs-devel@lists.sourceforge.net>,
        <linux-kernel@vger.kernel.org>, <chao@kernel.org>
References: <20190219081529.5106-1-yuchao0@huawei.com>
 <d2b3c101-0399-4e85-5765-7b6504aaca74@huawei.com>
 <20190901071757.GA49907@jaegeuk-macbookpro.roam.corp.google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <024fe351-8e25-35cd-47a7-9755498c73f4@huawei.com>
Date:   Mon, 2 Sep 2019 09:16:35 +0800
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On 2019/9/1 15:17, Jaegeuk Kim wrote:
> On 08/31, Chao Yu wrote:
>> On 2019/2/19 16:15, Chao Yu wrote:
>>> @@ -1976,10 +2035,13 @@ static int __write_data_page(struct page *page, bool *submitted,
>>>  	}
>>>  
>>>  	unlock_page(page);
>>> -	if (!S_ISDIR(inode->i_mode) && !IS_NOQUOTA(inode))
>>> +	if (!S_ISDIR(inode->i_mode) && !IS_NOQUOTA(inode)) {
>>> +		f2fs_submit_ipu_bio(sbi, bio, page);
>>>  		f2fs_balance_fs(sbi, need_balance_fs);
>>> +	}
>>
>> Above bio submission was added to avoid below deadlock:
>>
>> - __write_data_page
>>  - f2fs_do_write_data_page
>>   - set_page_writeback        ---- set writeback flag
>>   - f2fs_inplace_write_data
>>  - f2fs_balance_fs
>>   - f2fs_gc
>>    - do_garbage_collect
>>     - gc_data_segment
>>      - move_data_page
>>       - f2fs_wait_on_page_writeback
>>        - wait_on_page_writeback  --- wait writeback
>>
>> However, it breaks the merge of IPU IOs, to solve this issue, it looks we need
>> to add global bio cache for such IPU merge case, then later
>> f2fs_wait_on_page_writeback can check whether writebacked page is cached or not,
>> and do the submission if necessary.
>>
>> Jaegeuk, any thoughts?
> 
> How about calling f2fs_submit_ipu_bio() when we need to do GC in the same
> context?

However it also could happen in race case:

Thread A				Thread B
- __write_data_page (inode x, page y)
 - f2fs_do_write_data_page
  - set_page_writeback        ---- set writeback flag in page y
  - f2fs_inplace_write_data
 - f2fs_balance_fs
					 - lock gc_mutex
 - lock gc_mutex
					  - f2fs_gc
					   - do_garbage_collect
					    - gc_data_segment
					     - move_data_page
					      - f2fs_wait_on_page_writeback
					       - wait_on_page_writeback  --- wait writeback of page y

So it needs a global bio cache for merged IPU pages, how about adding a list to
link all ipu bios in struct f2fs_bio_info?

struct f2fs_bio_info {
	....
	struct list_head ipu_bio_list;	/* track all ipu bio */
	spinlock_t ipu_bio_lock;	/* protect ipu bio list */
}

> 
>>
>> Thanks,
> .
>