Re: [PATCH V2] writeback: fix race that cause writeback hung

[Jan Kara <jack@suse.cz>]

On Sat 17-08-13 11:36:03, Junxiao Bi wrote:
> There is a race between mark inode dirty and writeback thread,
> see the following scenario. In this case, writeback thread will
> not run though there is dirty_io.
> 
> __mark_inode_dirty()                                          bdi_writeback_workfn()
> 	...                                                       	...
> 	spin_lock(&inode->i_lock);
> 	...
> 	if (bdi_cap_writeback_dirty(bdi)) {
> 	    <<< assume wb has dirty_io, so wakeup_bdi is false.
> 	    <<< the following inode_dirty also have wakeup_bdi false.
> 	    if (!wb_has_dirty_io(&bdi->wb))
> 		    wakeup_bdi = true;
> 	}
> 	spin_unlock(&inode->i_lock);
> 	                                                            <<< assume last dirty_io is removed here.
> 	                                                            pages_written = wb_do_writeback(wb);
> 	                                                            ...
> 	                                                            <<< work_list empty and wb has no dirty_io,
> 	                                                            <<< delayed_work will not be queued.
> 	                                                            if (!list_empty(&bdi->work_list) ||
> 	                                                                (wb_has_dirty_io(wb) && dirty_writeback_interval))
> 	                                                                queue_delayed_work(bdi_wq, &wb->dwork,
> 	                                                                    msecs_to_jiffies(dirty_writeback_interval * 10));
> 	spin_lock(&bdi->wb.list_lock);
> 	inode->dirtied_when = jiffies;
> 	<<< new dirty_io is added.
> 	list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
> 	spin_unlock(&bdi->wb.list_lock);
> 
> 	<<< though there is dirty_io, but wakeup_bdi is false,
> 	<<< so writeback thread will not be waked up and
> 	<<< the new dirty_io will not be flushed.
> 	if (wakeup_bdi)
> 	    bdi_wakeup_thread_delayed(bdi);
> 
> Writeback will run until there is a new flush work queued.
> This may cause a lot of dirty pages stay in memory for a long time.
> 
> Cc: Jan Kara <jack@suse.cz>
> Cc: Fengguang Wu <fengguang.wu@intel.com>
> Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
  Looks good. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/fs-writeback.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 68851ff..053ec91 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1173,6 +1173,8 @@ void __mark_inode_dirty(struct inode *inode, int flags)
>  			bool wakeup_bdi = false;
>  			bdi = inode_to_bdi(inode);
>  
> +			spin_unlock(&inode->i_lock);
> +			spin_lock(&bdi->wb.list_lock);
>  			if (bdi_cap_writeback_dirty(bdi)) {
>  				WARN(!test_bit(BDI_registered, &bdi->state),
>  				     "bdi-%s not registered\n", bdi->name);
> @@ -1187,8 +1189,6 @@ void __mark_inode_dirty(struct inode *inode, int flags)
>  					wakeup_bdi = true;
>  			}
>  
> -			spin_unlock(&inode->i_lock);
> -			spin_lock(&bdi->wb.list_lock);
>  			inode->dirtied_when = jiffies;
>  			list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
>  			spin_unlock(&bdi->wb.list_lock);
> -- 
> 1.7.9.5
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR