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

Darrick J. Wong
2013-10-10T19:29:19Z

On Thu, Oct 10, 2013 at 05:53:37PM +0200, Lukáš Czerner wrote:
> 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.

Hmm. I think I could eliminate the middle ext2fs_map_cluster_block() calls by
only calling it if free_start is within cluster_ratio blocks of the pre-loop
value of free_start, or if free_count < cluster_ratio. I can also split the
whole thing into three loops (pre-cluster, clusters, and post-cluster), though
for the non-bigalloc case I'd skip to the middle loop.

--D
>
> Thanks!
> -Lukas
>
>
> > }
> > next_extent:
> > retval = ext2fs_extent_get(handle, op,
> >
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