Re: [f2fs-dev] [PATCH 2/2] f2fs: fix to avoid data update racing between GC and DIO

[Chao Yu <chao@kernel.org>]

On 2016/7/7 6:37, Jaegeuk Kim wrote:
> On Wed, Jul 06, 2016 at 10:10:57AM +0800, Chao Yu wrote:
>> On 2016/7/6 8:24, Jaegeuk Kim wrote:
>>> On Fri, Jul 01, 2016 at 02:03:17PM +0800, Chao Yu wrote:
>>>> Hi Jaegeuk,
>>>>
>>>> On 2016/7/1 8:03, Jaegeuk Kim wrote:
>>>>> Hi Chao,
>>>>>
>>>>> On Thu, Jun 30, 2016 at 04:42:48PM +0800, Chao Yu wrote:
>>>>>> Datas in file can be operated by GC and DIO simultaneously, so we will
>>>>>> face race case as below:
>>>>>>
>>>>>> For write case:
>>>>>> Thread A				Thread B
>>>>>> - generic_file_direct_write
>>>>>>  - invalidate_inode_pages2_range
>>>>>>  - f2fs_direct_IO
>>>>>>   - do_blockdev_direct_IO
>>>>>>    - do_direct_IO
>>>>>>     - get_more_blocks
>>>>>> 					- f2fs_gc
>>>>>> 					 - do_garbage_collect
>>>>>> 					  - gc_data_segment
>>>>>> 					   - move_data_page
>>>>>> 					    - do_write_data_page
>>>>>> 					    migrate data block to new block address
>>>>>>    - dio_bio_submit
>>>>>>    update user data to old block address
>>>>>>
>>>>>> For read case:
>>>>>> Thread A                                Thread B
>>>>>> - generic_file_direct_write
>>>>>>  - invalidate_inode_pages2_range
>>>>>>  - f2fs_direct_IO
>>>>>>   - do_blockdev_direct_IO
>>>>>>    - do_direct_IO
>>>>>>     - get_more_blocks
>>>>>> 					- f2fs_balance_fs
>>>>>> 					 - f2fs_gc
>>>>>> 					  - do_garbage_collect
>>>>>> 					   - gc_data_segment
>>>>>> 					    - move_data_page
>>>>>> 					     - do_write_data_page
>>>>>> 					     migrate data block to new block address
>>>>>> 					  - write_checkpoint
>>>>>> 					   - do_checkpoint
>>>>>> 					    - clear_prefree_segments
>>>>>> 					     - f2fs_issue_discard
>>>>>>                                              discard old block adress
>>>>>>    - dio_bio_submit
>>>>>>    update user buffer from obsolete block address
>>>>>>
>>>>>> In order to fix this, for one file, we should let DIO and GC getting exclusion
>>>>>> against with each other.
>>>>>>
>>>>>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
>>>>>> ---
>>>>>>  fs/f2fs/data.c  |  2 ++
>>>>>>  fs/f2fs/f2fs.h  |  1 +
>>>>>>  fs/f2fs/gc.c    | 14 +++++++++++++-
>>>>>>  fs/f2fs/super.c |  1 +
>>>>>>  4 files changed, 17 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>>> index ba4963f..08dc060 100644
>>>>>> --- a/fs/f2fs/data.c
>>>>>> +++ b/fs/f2fs/data.c
>>>>>> @@ -1716,7 +1716,9 @@ static ssize_t f2fs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>>>>>>  
>>>>>>  	trace_f2fs_direct_IO_enter(inode, offset, count, iov_iter_rw(iter));
>>>>>>  
>>>>>> +	mutex_lock(&F2FS_I(inode)->dio_mutex);
>>>>>>  	err = blockdev_direct_IO(iocb, inode, iter, get_data_block_dio);
>>>>>> +	mutex_unlock(&F2FS_I(inode)->dio_mutex);
>>>>>
>>>>> This means we need to sacrifice entire parallism even in the normal cases?
>>>>> Can we find another way?
>>>>
>>>> 1. For dio write vs dio write, writer will grab i_mutex before dio_mutex, so
>>>> anyway, concurrent dio writes will be exclusive.
>>>>
>>>> 2. For dio write vs gc, keep using dio_mutex for making them exclusive.
>>>>
>>>> 3. For dio read vs dio read, and dio read vs gc, what about adding dio_rwsem to
>>>> control the parallelism?
>>>>
>>>> 4. For dio write vs dio read, we grab different lock (write grabs dio_mutex,
>>>> read grabs dio_rwsem), so there is no race condition.
>>>
>>> How about adding a flag in a dio inode and avoiding GCs for there-in blocks?
>>
>> Hmm.. IMO, without lock, it's hard to keep the sequence that let GC checking the
>> flag after setting it, right?
> 
> Okay, could you add dio_rwsem for now?
> Later, we may need to take a look at dio_overwrite case to mitigate inode_lock
> contention likewise xfs. :)

Sounds good if we can support concurrent overwrite dio! :)

Let me send v3.

Thanks,

> 
> Thanks,
> 
>>>>>
>>>>>>  	if (iov_iter_rw(iter) == WRITE) {
>>>>>>  		if (err > 0)
>>>>>>  			set_inode_flag(inode, FI_UPDATE_WRITE);
>>>>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>>>>> index bd82b6d..a241576 100644
>>>>>> --- a/fs/f2fs/f2fs.h
>>>>>> +++ b/fs/f2fs/f2fs.h
>>>>>> @@ -474,6 +474,7 @@ struct f2fs_inode_info {
>>>>>>  	struct list_head inmem_pages;	/* inmemory pages managed by f2fs */
>>>>>>  	struct mutex inmem_lock;	/* lock for inmemory pages */
>>>>>>  	struct extent_tree *extent_tree;	/* cached extent_tree entry */
>>>>>> +	struct mutex dio_mutex;		/* avoid racing between dio and gc */
>>>>>>  };
>>>>>>  
>>>>>>  static inline void get_extent_info(struct extent_info *ext,
>>>>>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
>>>>>> index c2c4ac3..98e3763 100644
>>>>>> --- a/fs/f2fs/gc.c
>>>>>> +++ b/fs/f2fs/gc.c
>>>>>> @@ -744,12 +744,24 @@ next_step:
>>>>>>  		/* phase 3 */
>>>>>>  		inode = find_gc_inode(gc_list, dni.ino);
>>>>>>  		if (inode) {
>>>>>> +			bool locked = false;
>>>>>> +
>>>>>> +			if (S_ISREG(inode->i_mode)) {
>>>>>> +				if (!mutex_trylock(&F2FS_I(inode)->dio_mutex))
>>>>>> +					continue;
>>>>>> +				locked = true;
>>>>>> +			}
>>>>>> +
>>>>>>  			start_bidx = start_bidx_of_node(nofs, inode)
>>>>>>  								+ ofs_in_node;
>>>>>> -			if (f2fs_encrypted_inode(inode) && S_ISREG(inode->i_mode))
>>>>>> +			if (f2fs_encrypted_inode(inode) &&
>>>>>> +							S_ISREG(inode->i_mode))
>>>>>>  				move_encrypted_block(inode, start_bidx);
>>>>>>  			else
>>>>>>  				move_data_page(inode, start_bidx, gc_type);
>>>>>> +			if (locked)
>>>>>> +				mutex_unlock(&F2FS_I(inode)->dio_mutex);
>>>>>> +
>>>>>>  			stat_inc_data_blk_count(sbi, 1, gc_type);
>>>>>>  		}
>>>>>>  	}
>>>>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>>>>> index 8c698e1..24aab3f 100644
>>>>>> --- a/fs/f2fs/super.c
>>>>>> +++ b/fs/f2fs/super.c
>>>>>> @@ -575,6 +575,7 @@ static struct inode *f2fs_alloc_inode(struct super_block *sb)
>>>>>>  	INIT_LIST_HEAD(&fi->gdirty_list);
>>>>>>  	INIT_LIST_HEAD(&fi->inmem_pages);
>>>>>>  	mutex_init(&fi->inmem_lock);
>>>>>> +	mutex_init(&fi->dio_mutex);
>>>>>>  
>>>>>>  	/* Will be used by directory only */
>>>>>>  	fi->i_dir_level = F2FS_SB(sb)->dir_level;
>>>>>> -- 
>>>>>> 2.8.2.311.gee88674
>>>>>
>>>>> .
>>>>>
>>>
>>> .
>>>
> 
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