Re: [PATCH] fs: add sync_inode_metadata

[Jan Kara <jack@suse.cz>]

On Wed 06-10-10 10:48:20, Christoph Hellwig wrote:
> Add a new helper to write out the inode using the writeback code,
> that is including the correct dirty bit and list manipulation.  A few
> of filesystems already opencode this, and a lot of others should be
> using it instead of using write_inode_now which also writes out the
> data.

  I'm reacting late but anyways:
> Index: linux-2.6/fs/fs-writeback.c
> ===================================================================
> --- linux-2.6.orig/fs/fs-writeback.c	2010-10-05 10:51:24.611023726 +0200
> +++ linux-2.6/fs/fs-writeback.c	2010-10-05 10:53:24.684255876 +0200
> @@ -1209,3 +1209,23 @@ int sync_inode(struct inode *inode, stru
>  	return ret;
>  }
>  EXPORT_SYMBOL(sync_inode);
> +
> +/**
> + * sync_inode - write an inode to disk
> + * @inode: the inode to sync
> + * @wait: wait for I/O to complete.
> + *
> + * Write an inode to disk and adjust it's dirty state after completion.
> + *
> + * Note: only writes the actual inode, no associated data or other metadata.
> + */
> +int sync_inode_metadata(struct inode *inode, int wait)
> +{
> +	struct writeback_control wbc = {
> +		.sync_mode = wait ? WB_SYNC_ALL : WB_SYNC_NONE,
> +		.nr_to_write = 0, /* metadata-only */
  This is a catch in write_cache_pages() code. If you call it in
WB_SYNC_ALL mode, they will just ignore the .nr_to_write argument. So what
actually ensures that we do not write data is that .range_start and
.range_end are implicitely set to 0. In fact, write_cache_pages() will
write page with index 0 in that case. So setting .range_start to 1 would be
even better.
  Otherwise the patch looks OK.

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