Re: [f2fs-dev] [PATCH] f2fs: fix to avoid potential deadlock

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 01/27, Chao Yu wrote:
> Quoted from Jing Xia's report, there is a potential deadlock may happen
> between kworker and checkpoint as below:
> 
> [T:writeback]				[T:checkpoint]
> - wb_writeback
>  - blk_start_plug
> bio contains NodeA was plugged in writeback threads

I'm still trying to understand more precisely. So, how is it possible to
have bio having node write in this current context?

> 					- do_writepages  -- sync write inodeB, inc wb_sync_req[DATA]
> 					 - f2fs_write_data_pages
> 					  - f2fs_write_single_data_page -- write last dirty page
> 					   - f2fs_do_write_data_page
> 					    - set_page_writeback  -- clear page dirty flag and
> 					    PAGECACHE_TAG_DIRTY tag in radix tree
> 					    - f2fs_outplace_write_data
> 					     - f2fs_update_data_blkaddr
> 					      - f2fs_wait_on_page_writeback -- wait NodeA to writeback here
> 					   - inode_dec_dirty_pages
>  - writeback_sb_inodes
>   - writeback_single_inode
>    - do_writepages
>     - f2fs_write_data_pages -- skip writepages due to wb_sync_req[DATA]
>      - wbc->pages_skipped += get_dirty_pages() -- PAGECACHE_TAG_DIRTY is not set but get_dirty_pages() returns one
>   - requeue_inode -- requeue inode to wb->b_dirty queue due to non-zero.pages_skipped
>  - blk_finish_plug
> 
> Let's try to avoid deadlock condition by forcing unplugging previous bio via
> blk_finish_plug(current->plug) once we'v skipped writeback in writepages()
> due to valid sbi->wb_sync_req[DATA/NODE].
> 
> Fixes: 687de7f1010c ("f2fs: avoid IO split due to mixed WB_SYNC_ALL and WB_SYNC_NONE")
> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
> Signed-off-by: Jing Xia <jing.xia@unisoc.com>
> Signed-off-by: Chao Yu <chao@kernel.org>
> ---
>  fs/f2fs/data.c | 6 +++++-
>  fs/f2fs/node.c | 6 +++++-
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 76d6fe7b0c8f..932a4c81acaf 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -3174,8 +3174,12 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
>  	/* to avoid spliting IOs due to mixed WB_SYNC_ALL and WB_SYNC_NONE */
>  	if (wbc->sync_mode == WB_SYNC_ALL)
>  		atomic_inc(&sbi->wb_sync_req[DATA]);
> -	else if (atomic_read(&sbi->wb_sync_req[DATA]))
> +	else if (atomic_read(&sbi->wb_sync_req[DATA])) {
> +		/* to avoid potential deadlock */
> +		if (current->plug)
> +			blk_finish_plug(current->plug);
>  		goto skip_write;
> +	}
>  
>  	if (__should_serialize_io(inode, wbc)) {
>  		mutex_lock(&sbi->writepages);
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index 556fcd8457f3..69c6bcaf5aae 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -2106,8 +2106,12 @@ static int f2fs_write_node_pages(struct address_space *mapping,
>  
>  	if (wbc->sync_mode == WB_SYNC_ALL)
>  		atomic_inc(&sbi->wb_sync_req[NODE]);
> -	else if (atomic_read(&sbi->wb_sync_req[NODE]))
> +	else if (atomic_read(&sbi->wb_sync_req[NODE])) {
> +		/* to avoid potential deadlock */
> +		if (current->plug)
> +			blk_finish_plug(current->plug);
>  		goto skip_write;
> +	}
>  
>  	trace_f2fs_writepages(mapping->host, wbc, NODE);
>  
> -- 
> 2.32.0


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