linux-ext4 - Re: [patch] ext4: fix race between unwritten extent conversion and truncate

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Date:	Thu, 26 Jan 2012 22:15:26 +0100
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [patch] ext4: fix race between unwritten extent conversion and
 truncate

On Thu 26-01-12 15:59:11, Jeff Moyer wrote:
> Hi,
> 
> The following comment in ext4_end_io_dio caught my attention:
> 
> 	/* XXX: probably should move into the real I/O completion handler */
>         inode_dio_done(inode);
> 
> The truncate code takes i_mutex, then calls inode_dio_wait.  Because the
> ext4 code path above will end up dropping the mutex before it is
> reacquired by the worker thread that does the extent conversion, it
> seems to me that the truncate can happen out of order.  Jan Kara
> mentioned that this might result in extra journal I/O, which isn't nice,
  Funny misunderstanding ;) I meant we will complain to system log with error
messages / WARN_ON.

> but that's probably the full extent of the "damage."
> 
> The fix is pretty straight-forward: don't call inode_dio_done until the
> extent conversion is complete.
  Otherwise the patch looks good so:
Reviewed-by: Jan Kara <jack@...e.cz>

								Honza
> 
> Signed-off-by: Jeff Moyer <jmoyer@...hat.com>
> CC: Jan Kara <jack@...e.cz>
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 513004f..2d55d7c 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -184,6 +184,7 @@ struct mpage_da_data {
>  #define	EXT4_IO_END_UNWRITTEN	0x0001
>  #define EXT4_IO_END_ERROR	0x0002
>  #define EXT4_IO_END_QUEUED	0x0004
> +#define EXT4_IO_END_DIRECT	0x0008
>  
>  struct ext4_io_page {
>  	struct page	*p_page;
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index feaa82f..f6dc02b 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2795,9 +2795,6 @@ out:
>  
>  	/* queue the work to convert unwritten extents to written */
>  	queue_work(wq, &io_end->work);
> -
> -	/* XXX: probably should move into the real I/O completion handler */
> -	inode_dio_done(inode);
>  }
>  
>  static void ext4_end_io_buffer_write(struct buffer_head *bh, int uptodate)
> @@ -2921,9 +2918,12 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
>  		iocb->private = NULL;
>  		EXT4_I(inode)->cur_aio_dio = NULL;
>  		if (!is_sync_kiocb(iocb)) {
> -			iocb->private = ext4_init_io_end(inode, GFP_NOFS);
> -			if (!iocb->private)
> +			ext4_io_end_t *io_end =
> +				ext4_init_io_end(inode, GFP_NOFS);
> +			if (!io_end)
>  				return -ENOMEM;
> +			io_end->flag |= EXT4_IO_END_DIRECT;
> +			iocb->private = io_end;
>  			/*
>  			 * we save the io structure for current async
>  			 * direct IO, so that later ext4_map_blocks()
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 4758518..9e1b8eb 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -110,6 +110,8 @@ int ext4_end_io_nolock(ext4_io_end_t *io)
>  	if (io->iocb)
>  		aio_complete(io->iocb, io->result, 0);
>  
> +	if (io->flag & EXT4_IO_END_DIRECT)
> +		inode_dio_done(inode);
>  	/* Wake up anyone waiting on unwritten extent conversion */
>  	if (atomic_dec_and_test(&EXT4_I(inode)->i_aiodio_unwritten))
>  		wake_up_all(ext4_ioend_wq(io->inode));
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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