linux-kernel - Re: [PATCH -V8 2/9] vfs: Add name to file handle conversion support

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Date:	Wed, 19 May 2010 16:15:51 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Dave Chinner <david@...morbit.com>, hch@...radead.org,
	viro@...iv.linux.org.uk, adilger@....com, corbet@....net,
	serue@...ibm.com, neilb@...e.de, linux-fsdevel@...r.kernel.org,
	sfrench@...ibm.com, philippe.deniel@....FR,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V8 2/9] vfs: Add name to file handle conversion support


"Aneesh Kumar K. V":
> Now that we are not doing UUID based vfsmount lookup this make
> sense. Will update in the next iteration with UUID to be part of
> super_block.

Because this UUID is just for some FS's userspace helpers (in other
words, returning UUID is FS specific behaviour), I am afraid it is not a
good ideat to put the array into generic super_block.

About the design or approach, this might have been discussed earlier,
but I'd like to suggest to clarify some points here.
- While these new systemcalls provide generic features, the
  implementation depends upon s_export_op, ie. NFS-exporting.
  It means there are two requirements for these systemcalls, enabling
  CONFIG_EXPORTFS and FS has to set s_export_op.
  Is this acceptable?

- exportfs_encode_fh() supports the default encoding
  export_encode_fh(), but exportfs_decode_fh() doesn't.
  The latest patch series modifes exportfs_decode_fh() to return ESTALE,
  but I'd suggest to make the caller of export_encode_fh() to check
  s_export_op->fh_to_dentry() and return ENOSYS.
  Or implement the default decode routine as a contrast of
  export_encode_fh().

- Some FS (or its userspace helper) may want to put UUID into the
  handle. If those FS already have UUID in their fs private_data, then
  putting a pointer (instead of an array) is better.
  Or creating two new operations in s_op to encode/decode handle
  may be good too (regardless CONFIG_EXPORTFS). The generic
  implementations should be provided, and when these function pointers
  in s_op are not set, they should be called as default. These generic
  implementaions will be able to be used by expfs.c too. And UUID in
  super_block will be unnecessary.

If my English is broken and hard to understand, please let me know.


J. R. Okajima
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