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Date:   Fri, 14 Feb 2020 15:35:18 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Sahitya Tummala <stummala@...eaurora.org>
CC:     Jaegeuk Kim <jaegeuk@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix the panic in do_checkpoint()

On 2020/2/14 15:16, Chao Yu wrote:
> Hi Sahitya,
> 
> On 2020/2/14 11:54, Sahitya Tummala wrote:
>> Hi Chao,
>>
>> On Fri, Feb 14, 2020 at 10:26:18AM +0800, Chao Yu wrote:
>>> Hi Sahitya,
>>>
>>> On 2020/2/12 18:34, Sahitya Tummala wrote:
>>>> There could be a scenario where f2fs_sync_meta_pages() will not
>>>> ensure that all F2FS_DIRTY_META pages are submitted for IO. Thus,
>>>> resulting in the below panic in do_checkpoint() -
>>>>
>>>> f2fs_bug_on(sbi, get_pages(sbi, F2FS_DIRTY_META) &&
>>>> 				!f2fs_cp_error(sbi));
>>>>
>>>> This can happen in a low-memory condition, where shrinker could
>>>> also be doing the writepage operation (stack shown below)
>>>> at the same time when checkpoint is running on another core.
>>>>
>>>> schedule
>>>> down_write
>>>> f2fs_submit_page_write -> by this time, this page in page cache is tagged
>>>> 			as PAGECACHE_TAG_WRITEBACK and PAGECACHE_TAG_DIRTY
>>>> 			is cleared, due to which f2fs_sync_meta_pages()
>>>> 			cannot sync this page in do_checkpoint() path.
>>>> f2fs_do_write_meta_page
>>>> __f2fs_write_meta_page
>>>> f2fs_write_meta_page
>>>> shrink_page_list
>>>> shrink_inactive_list
>>>> shrink_node_memcg
>>>> shrink_node
>>>> kswapd
>>>
>>> IMO, there may be one more simple fix here:
>>>
>>> -	f2fs_do_write_meta_page(sbi, page, io_type);
>>> 	dec_page_count(sbi, F2FS_DIRTY_META);
>>>
>>> +	f2fs_do_write_meta_page(sbi, page, io_type);
>>>
>>> If we can remove F2FS_DIRTY_META reference count before we clear
>>> PAGECACHE_TAG_DIRTY, we can avoid this race condition.
>>>
>>> - dec_page_count(sbi, F2FS_DIRTY_META);
>>> - f2fs_do_write_meta_page
>>>  - set_page_writeback
>>>   - __test_set_page_writeback
>>>    - xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
>>>
>>> Thoughts?
>>
>> I believe it will be a problem because let's say the shrinker path is 
>> still in the below stack -
>>
>> f2fs_submit_page_write();
>> ->down_write(&io->io_rwsem);
>>
>> Then, the current f2fs_wait_on_all_pages_writeback() will not wait for
>> that page as it is not yet tagged as F2FS_WB_CP_DATA. Hence, the checkpoint
>> may proceed without waiting for this meta page to be written to disk.
> 
> Oh, you're right.
> 
> It looks the race can happen for data/node pages? like in fsync() procedure.

I mean something like this:

- fsync()				- shrink
 - f2fs_do_sync_file
					 - __write_node_page
					  - set_page_writeback(page#0)
					  : remove DIRTY/TOWRITE flag
  - f2fs_fsync_node_pages
  : won't find page #0 as TOWRITE flag was removeD
  - f2fs_wait_on_node_pages_writeback
  : wont' wait page #0 writeback as it was not in fsync_node_list list.
					   - f2fs_add_fsync_node_entry

Thanks,

> 
>>
>>>
>>>>
>>>> Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>
>>>> ---
>>>>  fs/f2fs/checkpoint.c | 16 ++++++++--------
>>>>  fs/f2fs/f2fs.h       |  2 +-
>>>>  fs/f2fs/super.c      |  2 +-
>>>>  3 files changed, 10 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>>>> index ffdaba0..2b651a3 100644
>>>> --- a/fs/f2fs/checkpoint.c
>>>> +++ b/fs/f2fs/checkpoint.c
>>>> @@ -1250,14 +1250,14 @@ static void unblock_operations(struct f2fs_sb_info *sbi)
>>>>  	f2fs_unlock_all(sbi);
>>>>  }
>>>>  
>>>> -void f2fs_wait_on_all_pages_writeback(struct f2fs_sb_info *sbi)
>>>> +void f2fs_wait_on_all_pages(struct f2fs_sb_info *sbi, int type)
>>>>  {
>>>>  	DEFINE_WAIT(wait);
>>>>  
>>>>  	for (;;) {
>>>>  		prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);
>>>>  
>>>> -		if (!get_pages(sbi, F2FS_WB_CP_DATA))
>>>> +		if (!get_pages(sbi, type))
>>>>  			break;
>>>>  
>>>>  		if (unlikely(f2fs_cp_error(sbi)))
>>>> @@ -1384,8 +1384,8 @@ static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>>>>  
>>>>  	/* Flush all the NAT/SIT pages */
>>>>  	f2fs_sync_meta_pages(sbi, META, LONG_MAX, FS_CP_META_IO);
>>>> -	f2fs_bug_on(sbi, get_pages(sbi, F2FS_DIRTY_META) &&
>>>> -					!f2fs_cp_error(sbi));
>>>> +	/* Wait for all dirty meta pages to be submitted for IO */
>>>> +	f2fs_wait_on_all_pages(sbi, F2FS_DIRTY_META);
>>>
>>> I'm afraid calling f2fs_wait_on_all_pages() after we call
>>> f2fs_sync_meta_pages() is low efficient, as we only want to write out
>>> dirty meta pages instead of wait for writebacking them to device cache.
>>>
>>
>> I have modified the existing function f2fs_wait_on_all_pages_writeback() to
>> a generic one f2fs_wait_on_all_pages(), where it will wait according to the
>> requested type. In this case, it will only wait for dirty F2FS_DIRTY_META pages
>> but not for the writeback to device cache.
> 
> Oh, I see, as last dirty reference count decreaser won't wake up waiter, we need
> to wait for timeout, right? Can we decrease timeout count, maybe HZ/50 as we did
> for congestion.
> 
> io_schedule_timeout(5*HZ);
> 
> Thanks,
> 
>>
>> Thanks,
>>
>>> Thanks,
>>>
>>>>  
>>>>  	/*
>>>>  	 * modify checkpoint
>>>> @@ -1493,11 +1493,11 @@ static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>>>>  
>>>>  	/* Here, we have one bio having CP pack except cp pack 2 page */
>>>>  	f2fs_sync_meta_pages(sbi, META, LONG_MAX, FS_CP_META_IO);
>>>> -	f2fs_bug_on(sbi, get_pages(sbi, F2FS_DIRTY_META) &&
>>>> -					!f2fs_cp_error(sbi));
>>>> +	/* Wait for all dirty meta pages to be submitted for IO */
>>>> +	f2fs_wait_on_all_pages(sbi, F2FS_DIRTY_META);
>>>>  
>>>>  	/* wait for previous submitted meta pages writeback */
>>>> -	f2fs_wait_on_all_pages_writeback(sbi);
>>>> +	f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA);
>>>>  
>>>>  	/* flush all device cache */
>>>>  	err = f2fs_flush_device_cache(sbi);
>>>> @@ -1506,7 +1506,7 @@ static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>>>>  
>>>>  	/* barrier and flush checkpoint cp pack 2 page if it can */
>>>>  	commit_checkpoint(sbi, ckpt, start_blk);
>>>> -	f2fs_wait_on_all_pages_writeback(sbi);
>>>> +	f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA);
>>>>  
>>>>  	/*
>>>>  	 * invalidate intermediate page cache borrowed from meta inode
>>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>>> index 5a888a0..b0e0535 100644
>>>> --- a/fs/f2fs/f2fs.h
>>>> +++ b/fs/f2fs/f2fs.h
>>>> @@ -3196,7 +3196,7 @@ bool f2fs_is_dirty_device(struct f2fs_sb_info *sbi, nid_t ino,
>>>>  void f2fs_update_dirty_page(struct inode *inode, struct page *page);
>>>>  void f2fs_remove_dirty_inode(struct inode *inode);
>>>>  int f2fs_sync_dirty_inodes(struct f2fs_sb_info *sbi, enum inode_type type);
>>>> -void f2fs_wait_on_all_pages_writeback(struct f2fs_sb_info *sbi);
>>>> +void f2fs_wait_on_all_pages(struct f2fs_sb_info *sbi, int type);
>>>>  int f2fs_write_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc);
>>>>  void f2fs_init_ino_entry_info(struct f2fs_sb_info *sbi);
>>>>  int __init f2fs_create_checkpoint_caches(void);
>>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>>> index 5111e1f..084633b 100644
>>>> --- a/fs/f2fs/super.c
>>>> +++ b/fs/f2fs/super.c
>>>> @@ -1105,7 +1105,7 @@ static void f2fs_put_super(struct super_block *sb)
>>>>  	/* our cp_error case, we can wait for any writeback page */
>>>>  	f2fs_flush_merged_writes(sbi);
>>>>  
>>>> -	f2fs_wait_on_all_pages_writeback(sbi);
>>>> +	f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA);
>>>>  
>>>>  	f2fs_bug_on(sbi, sbi->fsync_node_num);
>>>>  
>>>>
>>
> 
> 
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> .
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