Re: [RFC PATCH] ext4: Fix the locking with respect to ext3 to ext4 migrate.

[Mingming Cao <cmm@us.ibm.com>]

On Fri, 2008-03-07 at 16:23 +0530, Aneesh Kumar K.V wrote:
Hi Aneesh,

>  static int init_inodecache(void)
> diff --git a/include/linux/ext4_fs_i.h b/include/linux/ext4_fs_i.h
> index d5508d3..96c0b4f 100644
> --- a/include/linux/ext4_fs_i.h
> +++ b/include/linux/ext4_fs_i.h
> @@ -162,6 +162,18 @@ struct ext4_inode_info {
>  	/* mballoc */
>  	struct list_head i_prealloc_list;
>  	spinlock_t i_prealloc_lock;
> +
> +	/* When doing migrate we need to ensure that the i_data field
> +	 * doesn't change. With respect to write and truncate we can ensure
> +	 * the same by taking inode->i_mutex. But a write to mmap area
> +	 * mapping holes doesn't take i_mutex since it doesn't change the
> +	 * i_size. We also can't take i_data_sem because we would like to
> +	 * extend/restart the journal and locking order prevents us from
> +	 * restarting journal within i_data_sem. 

How about we start a journal with estimated worse case transaction
credits  and then take the i_data_sem down? So that we could ensure that
whenever the i_data_sem is hold, the i_data is protected. That is what
currently DIO does, I think. It would be nice to avoid introducing
another semaphore to protect i_data for migration if we could.

> This will be taken in
> +	 * page_mkwrite in the read mode and migrate will take it in the
> +	 * write mode.
> +	 */
> +	struct rw_semaphore i_migrate_sem;
>  };
> 
>  #endif	/* _LINUX_EXT4_FS_I */

Mingming