Re: [PATCH 1/4] btrfs-convert: make more use of cache_free_extents

[Sean Bartell <wingedtachikoma@gmail.com>]

On Tue, May 18, 2010 at 09:40:28PM +0800, Yan, Zheng  wrote:
> On Sat, Mar 20, 2010 at 12:24 PM, Sean Bartell
> <wingedtachikoma@gmail.com> wrote:
> > An extent_io_tree is used for all free space information. This allo=
ws
> > removal of ext2_alloc_block and ext2_free_block, and makes
> > create_ext2_image less ext2-specific.

> > + =A0 =A0 =A0 ret =3D ext2_cache_free_extents(ext2_fs, &orig_free_t=
ree);
> > + =A0 =A0 =A0 if (ret) {
> > + =A0 =A0 =A0 =A0 =A0 =A0 =A0 fprintf(stderr, "error during cache_f=
ree_extents %d\n", ret);
> > + =A0 =A0 =A0 =A0 =A0 =A0 =A0 goto fail;
> > + =A0 =A0 =A0 }
> > + =A0 =A0 =A0 /* preserve first 64KiB, just in case */
> > + =A0 =A0 =A0 clear_extent_dirty(&orig_free_tree, 0, BTRFS_SUPER_IN=
=46O_OFFSET - 1, 0);
> > +
> > + =A0 =A0 =A0 ret =3D copy_dirtiness(&free_tree, &orig_free_tree);
> > + =A0 =A0 =A0 if (ret) {
> > + =A0 =A0 =A0 =A0 =A0 =A0 =A0 fprintf(stderr, "error during copy_di=
rtiness %d\n", ret);
> > + =A0 =A0 =A0 =A0 =A0 =A0 =A0 goto fail;
> > + =A0 =A0 =A0 }

> extent_io_tree is not very space efficient. caching free space in
> several places is not good. I prefer adding a function that checks
> if a given block is used to the 'convert_fs' structure.

Good point. I'll change cache_free_extents to something like
  int (*iterate_used_extents)(struct convert_fs *fs, u64 start, u64 end=
,
                              void *priv, int (*cb)(u64 start, u64 end)=
)
create_image_file_range and do_convert should work well with a callback=
=2E

This also opens up the possibility of finding free extents
incrementally: call iterate_used_extents on the first GB, then
custom_alloc_extent will call it on the next GB once free space runs
out.
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