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

Jaegeuk Kim
2016-07-06T00:24:25Z

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?

Thanks,

>
> 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
> >
> > .
> >