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

Chao Yu
2016-07-01T06:08:36Z

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.

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
>
> .
>