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

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Date:	Wed, 19 May 2010 14:22:22 +0530
From:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	"J. R. Okajima" <hooanon05@...oo.co.jp>
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

On Wed, 19 May 2010 16:15:51 +0900, "J. R. Okajima" <hooanon05@...oo.co.jp> wrote:
> 
> "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.

UUID should be looked at as the file system identifier and IMHO struct
super_block is the right place to hold it. For ex: ext*  put then in
ext*_super_block. File system that doesn't support UUID can leave the
superblock field zero filled.


> 
> 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?


I think exportfs is the right interface we want to depend on for
generating a handle. We should not be having another parallel interface for file
handle generation. But agreed that we should return -EOPNOTSUPP in case
EXPORTFS is disabled.


> 
> - 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.

I will fix to make the syscall return EOPNOTSUPP in case fh_to_dentry is
not supported. But i guess we still need to keep the change in
exportfs_decode_fh to return -ESTALE in case these operations are not definied.



>   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.


IMHO that would be over design. We can depend on exportfs
interfaces and if not defined return EOPNOTSUPP.

-aneesh
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