ext4: fix race between unwritten extent conversion and truncate

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,
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.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
CC: Jan Kara <jack@suse.cz>

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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@suse.cz>

								Honza
> 
> Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
> CC: Jan Kara <jack@suse.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@suse.cz> writes:

> 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.

Heh.  Ted, let me know if I need to repost to fix that up...

Cheers,
Jeff
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