linux-ext4 - Re: [PATCH] ext4: fix ext4_flush_completed

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Date:	Thu, 4 Oct 2012 12:11:06 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org, tytso@....edu, jack@...e.cz
Subject: Re: [PATCH] ext4: fix ext4_flush_completed_IO wait semantics

On Wed 03-10-12 22:43:27, Dmitry Monakhov wrote:
> BUG #1) All places where we call ext4_flush_completed_IO are broken
>     because buffered io and DIO/AIO goes through three stages
>     1) submitted io,
>     2) completed io (in i_completed_io_list) conversion pended
>     3) finished  io (conversion done)
>     And by calling ext4_flush_completed_IO we will flush only
>     requests which were in (2) stage, which is wrong because:
>      1) punch_hole and truncate _must_ wait for all outstanding unwritten io
>       regardless to it's state.
>      2) fsync and nolock_dio_read should also wait because there is
>         a time window between end_page_writeback() and ext4_add_complete_io()
>         As result integrity fsync is broken in case of buffered write
>         to fallocated region:
>         fsync                                      blkdev_completion
> 	 ->filemap_write_and_wait_range
>                                                    ->ext4_end_bio
>                                                      ->end_page_writeback
>           <-- filemap_write_and_wait_range return
> 	 ->ext4_flush_completed_IO
>    	 sees empty i_completed_io_list but pended
>    	 conversion still exist
>                                                      ->ext4_add_complete_io
> 
> BUG #2) Race window becomes wider due to 'ext4: completed_io locking cleanup V4'
> 
> This patch make following changes:
> 1) ext4_flush_completed_io() now first try to flush completed io and when
>    wait for any outstanding unwritten io via ext4_unwritten_wait()
> 2) Rename function to more appropriate name.
> 3) Assert that all callers of ext4_flush_unwritten_io should hold i_mutex to
>    prevent endless wait
> 
> Signed-off-by: Dmitry Monakhov <dmonakhov@...nvz.org>
  This patch looks good except for:

> diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> index 8d849da..37cd5a4 100644
> --- a/fs/ext4/indirect.c
> +++ b/fs/ext4/indirect.c
> @@ -807,9 +807,11 @@ ssize_t ext4_ind_direct_IO(int rw, struct kiocb *iocb,
>  
>  retry:
>  	if (rw == READ && ext4_should_dioread_nolock(inode)) {
> -		if (unlikely(!list_empty(&ei->i_completed_io_list)))
> -			ext4_flush_completed_IO(inode);
> -
> +		if (unlikely(!atomic_read(&EXT4_I(inode)->i_unwritten))) {
  This condition which seems to be inverted...

> +			mutex_lock(&inode->i_mutex);
> +			ext4_flush_unwritten_io(inode);
> +			mutex_unlock(&inode->i_mutex);
> +		}
>  		/*
>  		 * Nolock dioread optimization may be dynamically disabled
>  		 * via ext4_inode_block_unlocked_dio(). Check inode's state

  After fixing that, you can add:
Reviewed-by: Jan Kara <jack@...e.cz>

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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