Re: [PATCH 22/31] libext2fs: During punch, only free a cluster if we're sure that all blocks in the cluster are being punched

Lukáš Czerner
2013-10-10T15:53:45Z

On Mon, 30 Sep 2013, Darrick J. Wong wrote:

> Date: Mon, 30 Sep 2013 18:29:03 -0700
> From: Darrick J. Wong <darrick.wong@oracle.com>
> To: tytso@mit.edu, darrick.wong@oracle.com
> Cc: linux-ext4@vger.kernel.org
> Subject: [PATCH 22/31] libext2fs: During punch,
> only free a cluster if we're sure that all blocks in the cluster are being
> punched
>
> When bigalloc is enabled, using ext2fs_block_alloc_stats2() to free any block
> in a cluster has the effect of freeing the entire cluster. This is problematic
> if a caller instructs us to punch, say, blocks 12-15 of a 16-block cluster,
> because blocks 0-11 now point to a "free" cluster.
>
> The naive way to solve this problem is to see if any of the other blocks in
> this logical cluster map to a physical cluster. If so, then we know that the
> cluster is still in use and it mustn't be freed. Otherwise, we are punching
> the last mapped block in this cluster, so we can free the cluster.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> lib/ext2fs/bmap.c | 28 ++++++++++++++++++++++++++++
> lib/ext2fs/ext2fs.h | 3 +++
> lib/ext2fs/punch.c | 30 +++++++++++++++++++++++++-----
> 3 files changed, 56 insertions(+), 5 deletions(-)
>
>
> diff --git a/lib/ext2fs/bmap.c b/lib/ext2fs/bmap.c
> index 5074587..a6e35a9 100644
> --- a/lib/ext2fs/bmap.c
> +++ b/lib/ext2fs/bmap.c
> @@ -173,6 +173,34 @@ static errcode_t implied_cluster_alloc(ext2_filsys fs, ext2_ino_t ino,
> return 0;
> }
>
> +errcode_t ext2fs_map_cluster_block(ext2_filsys fs, ext2_ino_t ino,
> + struct ext2_inode *inode, blk64_t lblk,
> + blk64_t *pblk)
> +{
> + ext2_extent_handle_t handle;
> + errcode_t retval;
> +
> + /* Need bigalloc and extents to be enabled */
> + *pblk = 0;
> + if (!EXT2_HAS_RO_COMPAT_FEATURE(fs->super,
> + EXT4_FEATURE_RO_COMPAT_BIGALLOC) ||
> + !(inode->i_flags & EXT4_EXTENTS_FL))
> + return 0;
> +
> + retval = ext2fs_extent_open2(fs, ino, inode, &handle);
> + if (retval)
> + goto out;
> +
> + retval = implied_cluster_alloc(fs, ino, inode, handle, lblk, pblk);
> + if (retval)
> + goto out2;
> +
> +out2:
> + ext2fs_extent_free(handle);
> +out:
> + return retval;
> +}
> +
> static errcode_t extent_bmap(ext2_filsys fs, ext2_ino_t ino,
> struct ext2_inode *inode,
> ext2_extent_handle_t handle,
> diff --git a/lib/ext2fs/ext2fs.h b/lib/ext2fs/ext2fs.h
> index 9fef6d3..88da8db 100644
> --- a/lib/ext2fs/ext2fs.h
> +++ b/lib/ext2fs/ext2fs.h
> @@ -925,6 +925,9 @@ extern errcode_t ext2fs_bmap2(ext2_filsys fs, ext2_ino_t ino,
> struct ext2_inode *inode,
> char *block_buf, int bmap_flags, blk64_t block,
> int *ret_flags, blk64_t *phys_blk);
> +errcode_t ext2fs_map_cluster_block(ext2_filsys fs, ext2_ino_t ino,
> + struct ext2_inode *inode, blk64_t lblk,
> + blk64_t *pblk);
>
> #if 0
> /* bmove.c */
> diff --git a/lib/ext2fs/punch.c b/lib/ext2fs/punch.c
> index 4471f46..e0193b0 100644
> --- a/lib/ext2fs/punch.c
> +++ b/lib/ext2fs/punch.c
> @@ -183,8 +183,8 @@ static errcode_t ext2fs_punch_extent(ext2_filsys fs, ext2_ino_t ino,
> ext2_extent_handle_t handle = 0;
> struct ext2fs_extent extent;
> errcode_t retval;
> - blk64_t free_start, next;
> - __u32 free_count, newlen;
> + blk64_t free_start, next, lfree_start, pblk;
> + __u32 free_count, newlen, cluster_freed;
> int freed = 0;
> int op;
>
> @@ -210,6 +210,7 @@ static errcode_t ext2fs_punch_extent(ext2_filsys fs, ext2_ino_t ino,
> /* Start of deleted region before extent;
> adjust beginning of extent */
> free_start = extent.e_pblk;
> + lfree_start = extent.e_lblk;
> if (next > end)
> free_count = end - extent.e_lblk + 1;
> else
> @@ -225,6 +226,7 @@ static errcode_t ext2fs_punch_extent(ext2_filsys fs, ext2_ino_t ino,
> dbg_printf("Case #%d\n", 2);
> newlen = start - extent.e_lblk;
> free_start = extent.e_pblk + newlen;
> + lfree_start = extent.e_lblk + newlen;
> free_count = extent.e_len - newlen;
> extent.e_len = newlen;
> } else {
> @@ -240,6 +242,7 @@ static errcode_t ext2fs_punch_extent(ext2_filsys fs, ext2_ino_t ino,
>
> extent.e_len = start - extent.e_lblk;
> free_start = extent.e_pblk + extent.e_len;
> + lfree_start = extent.e_lblk + extent.e_len;
> free_count = end - start + 1;
>
> dbg_print_extent("inserting", &newex);
> @@ -280,9 +283,26 @@ static errcode_t ext2fs_punch_extent(ext2_filsys fs, ext2_ino_t ino,
> goto errout;
> dbg_printf("Free start %llu, free count = %u\n",
> free_start, free_count);
> - while (free_count-- > 0) {
> - ext2fs_block_alloc_stats2(fs, free_start++, -1);
> - freed++;
> + while (free_count > 0) {
> + retval = ext2fs_map_cluster_block(fs, ino, inode,
> + lfree_start, &pblk);
> + if (retval)
> + goto errout;
> + if (!pblk) {
> + ext2fs_block_alloc_stats2(fs, free_start, -1);
> + freed++;
> + cluster_freed = EXT2FS_CLUSTER_RATIO(fs) -
> + (free_start & EXT2FS_CLUSTER_MASK(fs));
> + if (cluster_freed > free_count)
> + cluster_freed = free_count;
> + free_count -= cluster_freed;
> + free_start += cluster_freed;
> + lfree_start += cluster_freed;
> + continue;
> + }
> + free_count--;
> + free_start++;
> + lfree_start++;

I think that this is a little bit excessive. What I think we should
do here is to identify first and last partial cluster and possibly
call ext2fs_map_cluster_block() for those since we might or might
not want to free then depending on whether there are other blocks in
it in-use.

Then just iterate over the whole clusters in between and free them
all. Having to call ext2fs_map_cluster_block() for every single
block we're freeing from the extent tree is not really necessary I think
especially since we really need to get this information for those
possibly partial clusters at the start and end of the extent.

Thanks!
-Lukas

> }
> next_extent:
> retval = ext2fs_extent_get(handle, op,
>
> --
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