linux-kernel - Re: [PATCH] incorrect direct io error handling

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Message-ID: <20061218221515.GN44411608@melbourne.sgi.com>
Date:	Tue, 19 Dec 2006 09:15:15 +1100
From:	David Chinner <dgc@....com>
To:	Dmitriy Monakhov <dmonakhov@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, devel@...nvz.org,
	Andrew Morton <akpm@...l.org>, xfs@....sgi.com
Subject: Re: [PATCH] incorrect direct io error handling

On Mon, Dec 18, 2006 at 04:22:44PM +0300, Dmitriy Monakhov wrote:
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 8332c77..7c571dd 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2044,8 +2044,9 @@ generic_file_direct_write(struct kiocb *
>  	/*
>  	 * Sync the fs metadata but not the minor inode changes and
>  	 * of course not the data as we did direct DMA for the IO.
> -	 * i_mutex is held, which protects generic_osync_inode() from
> -	 * livelocking.  AIO O_DIRECT ops attempt to sync metadata here.
> +	 * i_mutex may not being held (XFS does this), if so some specific locking
> +	 * ordering must protect generic_osync_inode() from livelocking.
> +	 * AIO O_DIRECT ops attempt to sync metadata here.
>  	 */
>  	if ((written >= 0 || written == -EIOCBQUEUED) &&
>  	    ((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
> @@ -2279,6 +2280,17 @@ __generic_file_aio_write_nolock(struct k
>  
>  		written = generic_file_direct_write(iocb, iov, &nr_segs, pos,
>  							ppos, count, ocount);
> +		/*
> +		 * If host is not S_ISBLK generic_file_direct_write() may 
> +		 * have instantiated a few blocks outside i_size  files
> +		 * Trim these off again.
> +		 */
> +		if (unlikely(written < 0) && !S_ISBLK(inode->i_mode)) {
> +			loff_t isize = i_size_read(inode);
> +			if (pos + count > isize)
> +				vmtruncate(inode, isize);
> +		}
> +
>  		if (written < 0 || written == count)
>  			goto out;

You comment in the first hunk that i_mutex may not be held here,
but there's no comment in __generic_file_aio_write_nolock() that the
i_mutex must be held for !S_ISBLK devices.

> @@ -2341,6 +2353,13 @@ ssize_t generic_file_aio_write_nolock(st
>  	ssize_t ret;
>  
>  	BUG_ON(iocb->ki_pos != pos);
> +	/*
> +	 *  generic_file_buffered_write() may be called inside 
> +	 *  __generic_file_aio_write_nolock() even in case of
> +	 *  O_DIRECT for non S_ISBLK files. So i_mutex must be held.
> +	 */
> +	if (!S_ISBLK(inode->i_mode))
> +		BUG_ON(!mutex_is_locked(&inode->i_mutex));
>  
>  	ret = __generic_file_aio_write_nolock(iocb, iov, nr_segs,
>  			&iocb->ki_pos);

I note that you comment here in generic_file_aio_write_nolock(),
but it's not immediately obvious that this is refering to the
vmtruncate() call in __generic_file_aio_write_nolock().

IOWs, wouldn't it be better to put this comment and check in
__generic_file_aio_write_nolock() directly above the vmtruncate()
call that cares about this?

> @@ -2383,8 +2402,8 @@ ssize_t generic_file_aio_write(struct ki
>  EXPORT_SYMBOL(generic_file_aio_write);
>  
>  /*
> - * Called under i_mutex for writes to S_ISREG files.   Returns -EIO if something
> - * went wrong during pagecache shootdown.
> + * May be called without i_mutex for writes to S_ISREG files. XFS does this.
> + * Returns -EIO if something went wrong during pagecache shootdown.
>   */

Not sure you need to say "XFS does this" - other filesystems may do this
in the future.....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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