Re: [patch 4/8] fs, exportfs: Add export_encode_inode_fh helper

On Thu, Aug 16, 2012 at 04:38:14PM +0400, Cyrill Gorcunov wrote:

On Thu, Aug 16, 2012 at 10:24:48AM +0400, Cyrill Gorcunov wrote:

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On the other hand, if you want a real filehandle then wouldn't you want
to e.g. call the filesystem's ->encode_fh() if necessary, as
exportfs_encode_fh() does?

Well, one of the problem I hit when I've been trying to use encode_fh
is that every new implementation of encode_fh will require some size
(even unknown) in buffer where encoded data pushed. Correct me please
if I'm wrong. But with export_encode_inode_fh there is a small buffer
with pretty known size needed on stack needed for printing data in
fdinfo.

You can just give encode_fh a too-small data and let it fail if it's not
big enough.

(In practice I think everyone supports NFSv3 filehandles which have a
maximum size of 64 bytes.)

I'll think about it, thanks!

Hi Bruce, thinking a bit more I guess using general encode_fh is not that
convenient since it operates with dentries while our fdinfo output deals
with inodes. Thus I should either provide some new encode_fh variant
which would deal with inodes directly without "parents".

I can't see why that wouldn't work.

Which doesn't
look for me anyhow better than the new export_encode_inode_fh helper.

That isn't going to work for filesystems that define their own
encode_fh:

	$ git grep '\.encode_fh'
	fs/btrfs/export.c:      .encode_fh      = btrfs_encode_fh,
	fs/ceph/export.c:       .encode_fh = ceph_encode_fh,
	fs/fat/inode.c: .encode_fh      = fat_encode_fh,
	fs/fuse/inode.c:        .encode_fh      = fuse_encode_fh,
	fs/gfs2/export.c:       .encode_fh = gfs2_encode_fh,
	fs/isofs/export.c:      .encode_fh      = isofs_export_encode_fh,
	fs/nilfs2/namei.c:      .encode_fh = nilfs_encode_fh,
	fs/ocfs2/export.c:      .encode_fh      = ocfs2_encode_fh,
	fs/reiserfs/super.c:    .encode_fh = reiserfs_encode_fh,
	fs/udf/namei.c: .encode_fh      = udf_encode_fh,
	fs/xfs/xfs_export.c:    .encode_fh              = xfs_fs_encode_fh,
	mm/shmem.c:     .encode_fh      = shmem_encode_fh,

--b.

After all, if the use of encode_fh become a mandatory rule we can easily
extend fsnotify fdinfo output to support new scheme without breaking
user space, because output looks like

 | fhandle-type:        1 f_handle: 49b1060023552153

(ie if something is changed than these fields will be simply updated).

Or maybe I miss something?

	Cyrill