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

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Date:	Wed, 6 Jul 2016 15:37:58 -0700
From:	Jaegeuk Kim <jaegeuk@...nel.org>
To:	Chao Yu <yuchao0@...wei.com>
Cc:	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] f2fs: fix to avoid data update racing between GC and
 DIO

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@...wei.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. :)

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
> >>>
> >>> .
> >>>
> > 
> > .
> >