linux-ext4 - Re: [PATCH 6/6] libext2fs: try to roll back when splitting an extent fails

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Date:	Tue, 21 Jan 2014 15:49:46 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 6/6] libext2fs: try to roll back when splitting an extent
 fails

On Mon, Jan 20, 2014 at 10:22:17PM -0800, Darrick J. Wong wrote:
> If a client asks us to remap a block in the middle of an extent, we
> potentially have to allocate a fair number of blocks to handle extent
> tree splits.  A failure in either of the ext2fs_extent_insert calls
> leaves us with an extent tree that no longer maps the logical block in
> question and everything that came after it!  Therefore, try to roll
> back the extent tree changes before returning an error code.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>

Reviewed-by: Zheng Liu <wenqing.lz@...bao.com>
                                                - Zheng

> ---
>  lib/ext2fs/extent.c |   25 ++++++++++++++++++++++---
>  1 file changed, 22 insertions(+), 3 deletions(-)
> 
> 
> diff --git a/lib/ext2fs/extent.c b/lib/ext2fs/extent.c
> index 2928695..87921e6 100644
> --- a/lib/ext2fs/extent.c
> +++ b/lib/ext2fs/extent.c
> @@ -1438,13 +1438,17 @@ errcode_t ext2fs_extent_set_bmap(ext2_extent_handle_t handle,
>  			goto done;
>  	} else {
>  		__u32	orig_length;
> +		blk64_t	orig_lblk;
> +		struct ext2fs_extent orig_extent;
> +		errcode_t r2;
>  
>  #ifdef DEBUG
>  		printf("(re/un)mapping in middle of extent\n");
>  #endif
>  		/* need to split this extent; later */
> -
> +		orig_lblk = extent.e_lblk;
>  		orig_length = extent.e_len;
> +		orig_extent = extent;
>  
>  		/* shorten pre-split extent */
>  		extent.e_len = (logical - extent.e_lblk);
> @@ -1456,8 +1460,13 @@ errcode_t ext2fs_extent_set_bmap(ext2_extent_handle_t handle,
>  			/* insert new extent after current */
>  			retval = ext2fs_extent_insert(handle,
>  					EXT2_EXTENT_INSERT_AFTER, &newextent);
> -			if (retval)
> +			if (retval) {
> +				r2 = ext2fs_extent_goto(handle, orig_lblk);
> +				if (r2 == 0)
> +					ext2fs_extent_replace(handle, 0,
> +							      &orig_extent);
>  				goto done;
> +			}
>  		}
>  		/* add post-split extent */
>  		extent.e_pblk += extent.e_len + 1;
> @@ -1465,8 +1474,18 @@ errcode_t ext2fs_extent_set_bmap(ext2_extent_handle_t handle,
>  		extent.e_len = orig_length - extent.e_len - 1;
>  		retval = ext2fs_extent_insert(handle,
>  				EXT2_EXTENT_INSERT_AFTER, &extent);
> -		if (retval)
> +		if (retval) {
> +			if (physical) {
> +				r2 = ext2fs_extent_goto(handle,
> +							newextent.e_lblk);
> +				if (r2 == 0)
> +					ext2fs_extent_delete(handle, 0);
> +			}
> +			r2 = ext2fs_extent_goto(handle, orig_lblk);
> +			if (r2 == 0)
> +				ext2fs_extent_replace(handle, 0, &orig_extent);
>  			goto done;
> +		}
>  	}
>  
>  done:
> 
> --
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