Re: [PATCH] fs-writeback: do not requeue a clean inode having skipped pages

[Jan Kara]

On Fri 15-09-23 22:51:31, Chunhai Guo wrote:
> When writing back an inode and performing an fsync on it concurrently, a
> deadlock issue may arise as shown below. In each writeback iteration, a
> clean inode is requeued to the wb->b_dirty queue due to non-zero
> pages_skipped, without anything actually being written. This causes an
> infinite loop and prevents the plug from being flushed, resulting in a
> deadlock. We now avoid requeuing the clean inode to prevent this issue.
> 
>     wb_writeback        fsync (inode-Y)
> blk_start_plug(&plug)
> for (;;) {
>   iter i-1: some reqs with page-X added into plug->mq_list // f2fs node page-X with PG_writeback
>                         filemap_fdatawrite
>                           __filemap_fdatawrite_range // write inode-Y with sync_mode WB_SYNC_ALL
>                            do_writepages
>                             f2fs_write_data_pages
>                              __f2fs_write_data_pages // wb_sync_req[DATA]++ for WB_SYNC_ALL
>                               f2fs_write_cache_pages
>                                f2fs_write_single_data_page
>                                 f2fs_do_write_data_page
>                                  f2fs_outplace_write_data
>                                   f2fs_update_data_blkaddr
>                                    f2fs_wait_on_page_writeback
>                                      wait_on_page_writeback // wait for f2fs node page-X
>   iter i:
>     progress = __writeback_inodes_wb(wb, work)
>     . writeback_sb_inodes
>     .   __writeback_single_inode // write inode-Y with sync_mode WB_SYNC_NONE
>     .   . do_writepages
>     .   .   f2fs_write_data_pages
>     .   .   .  __f2fs_write_data_pages // skip writepages due to (wb_sync_req[DATA]>0)
>     .   .   .   wbc->pages_skipped += get_dirty_pages(inode) // wbc->pages_skipped = 1
>     .   if (!(inode->i_state & I_DIRTY_ALL)) // i_state = I_SYNC | I_SYNC_QUEUED
>     .    total_wrote++;  // total_wrote = 1
>     .   requeue_inode // requeue inode-Y to wb->b_dirty queue due to non-zero pages_skipped
>     if (progress) // progress = 1
>       continue;
>   iter i+1:
>       queue_io
>       // similar process with iter i, infinite for-loop !
> }
> blk_finish_plug(&plug)   // flush plug won't be called
> 
> Signed-off-by: Chunhai Guo <guochunhai@xxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 969ce991b0b0..c1af01b2c42d 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1535,10 +1535,15 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
>  
>  	if (wbc->pages_skipped) {
>  		/*
> -		 * writeback is not making progress due to locked
> -		 * buffers. Skip this inode for now.
> +		 * Writeback is not making progress due to locked buffers.
> +		 * Skip this inode for now. Although having skipped pages
> +		 * is odd for clean inodes, it can happen for some
> +		 * filesystems so handle that gracefully.
>  		 */
> -		redirty_tail_locked(inode, wb);
> +		if (inode->i_state & I_DIRTY_ALL)
> +			redirty_tail_locked(inode, wb);
> +		else
> +			inode_cgwb_move_to_attached(inode, wb);
>  		return;
>  	}
>  
> -- 
> 2.25.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR