linux-kernel - Re: [PATCH V2] writeback: Avoid skipping inode writeback

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Date:   Tue, 10 May 2022 12:06:06 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jing Xia <jing.xia@...soc.com>
Cc:     viro@...iv.linux.org.uk, jack@...e.cz, hch@....de,
        jing.xia.mail@...il.com, stable@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] writeback: Avoid skipping inode writeback

On Tue 10-05-22 10:35:14, Jing Xia wrote:
> We have run into an issue that a task gets stuck in
> balance_dirty_pages_ratelimited() when perform I/O stress testing.
> The reason we observed is that an I_DIRTY_PAGES inode with lots
> of dirty pages is in b_dirty_time list and standard background
> writeback cannot writeback the inode.
> After studing the relevant code, the following scenario may lead
> to the issue:
> 
> task1                                   task2
> -----                                   -----
> fuse_flush
>  write_inode_now //in b_dirty_time
>   writeback_single_inode
>    __writeback_single_inode
>                                  fuse_write_end
>                                   filemap_dirty_folio
>                                    __xa_set_mark:PAGECACHE_TAG_DIRTY
>     lock inode->i_lock
>     if mapping tagged PAGECACHE_TAG_DIRTY
>     inode->i_state |= I_DIRTY_PAGES
>     unlock inode->i_lock
>                                    __mark_inode_dirty:I_DIRTY_PAGES
>                                       lock inode->i_lock
>                                       -was dirty,inode stays in
>                                       -b_dirty_time
>                                       unlock inode->i_lock
> 
>    if(!(inode->i_state & I_DIRTY_All))
>       -not true,so nothing done
> 
> This patch moves the dirty inode to b_dirty list when the inode
> currently is not queued in b_io or b_more_io list at the end of
> writeback_single_inode.
> 
> Reviewed-by: Jan Kara <jack@...e.cz>
> Reviewed-by: Christoph Hellwig <hch@....de>
> CC: stable@...r.kernel.org
> Fixes: 0ae45f63d4ef ("vfs: add support for a lazytime mount option")
> Signed-off-by: Jing Xia <jing.xia@...soc.com>

Thanks. I've queued the patch to my tree and will push it to Linus tomorrow
or so.

								Honza

> ---
>  fs/fs-writeback.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 591fe9cf1659..1fae0196292a 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1712,6 +1712,10 @@ static int writeback_single_inode(struct inode *inode,
>  	 */
>  	if (!(inode->i_state & I_DIRTY_ALL))
>  		inode_cgwb_move_to_attached(inode, wb);
> +	else if (!(inode->i_state & I_SYNC_QUEUED) &&
> +		 (inode->i_state & I_DIRTY))
> +		redirty_tail_locked(inode, wb);
> +
>  	spin_unlock(&wb->list_lock);
>  	inode_sync_complete(inode);
>  out:
> -- 
> 2.17.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR