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exportfs: update documentation

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Update documentation to the current state of affairs.  Remove duplicated
method descruptions in exportfs.h and point to Documentation/filesystems/
Exporting instead.  Add a little file header comment in expfs.c describing
what's going on and mentioning Neils and my copyright [1].

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Neil Brown <neilb@suse.de>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: <linux-ext4@vger.kernel.org>
Cc: Dave Kleikamp <shaggy@austin.ibm.com>
Cc: Anton Altaparmakov <aia21@cantab.net>
Cc: David Chinner <dgc@sgi.com>
Cc: Timothy Shimmin <tes@sgi.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Chris Mason <mason@suse.com>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: "Vladimir V. Saveliev" <vs@namesys.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Mark Fasheh <mark.fasheh@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Christoph Hellwig 2007-10-21 16:42:19 -07:00 committed by Linus Torvalds
parent 3965516440
commit e38f981758
3 changed files with 65 additions and 107 deletions
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105
Documentation/filesystems/Exporting
View file

@ -2,9 +2,12 @@
Making Filesystems Exportable Making Filesystems Exportable
============================= =============================
Most filesystem operations require a dentry (or two) as a starting Overview
--------
All filesystem operations require a dentry (or two) as a starting
point. Local applications have a reference-counted hold on suitable point. Local applications have a reference-counted hold on suitable
dentrys via open file descriptors or cwd/root. However remote dentries via open file descriptors or cwd/root. However remote
applications that access a filesystem via a remote filesystem protocol applications that access a filesystem via a remote filesystem protocol
such as NFS may not be able to hold such a reference, and so need a such as NFS may not be able to hold such a reference, and so need a
different way to refer to a particular dentry. As the alternative different way to refer to a particular dentry. As the alternative
@ -13,14 +16,14 @@ server-reboot (among other things, though these tend to be the most
problematic), there is no simple answer like 'filename'. problematic), there is no simple answer like 'filename'.
The mechanism discussed here allows each filesystem implementation to The mechanism discussed here allows each filesystem implementation to
specify how to generate an opaque (out side of the filesystem) byte specify how to generate an opaque (outside of the filesystem) byte
string for any dentry, and how to find an appropriate dentry for any string for any dentry, and how to find an appropriate dentry for any
given opaque byte string. given opaque byte string.
This byte string will be called a "filehandle fragment" as it This byte string will be called a "filehandle fragment" as it
corresponds to part of an NFS filehandle. corresponds to part of an NFS filehandle.
A filesystem which supports the mapping between filehandle fragments A filesystem which supports the mapping between filehandle fragments
and dentrys will be termed "exportable". and dentries will be termed "exportable".
@ -89,11 +92,9 @@ For a filesystem to be exportable it must:
1/ provide the filehandle fragment routines described below. 1/ provide the filehandle fragment routines described below.
2/ make sure that d_splice_alias is used rather than d_add 2/ make sure that d_splice_alias is used rather than d_add
when ->lookup finds an inode for a given parent and name. when ->lookup finds an inode for a given parent and name.
Typically the ->lookup routine will end: Typically the ->lookup routine will end with a:
if (inode)
return d_splice(inode, dentry); return d_splice_alias(inode, dentry);
d_add(dentry, inode);
return NULL;
} }
@ -101,67 +102,39 @@ For a filesystem to be exportable it must:
A file system implementation declares that instances of the filesystem A file system implementation declares that instances of the filesystem
are exportable by setting the s_export_op field in the struct are exportable by setting the s_export_op field in the struct
super_block. This field must point to a "struct export_operations" super_block. This field must point to a "struct export_operations"
struct which could potentially be full of NULLs, though normally at struct which has the following members:
least get_parent will be set.
The primary operations are decode_fh and encode_fh. encode_fh (optional)
decode_fh takes a filehandle fragment and tries to find or create a Takes a dentry and creates a filehandle fragment which can later be used
dentry for the object referred to by the filehandle. to find or create a dentry for the same object. The default
encode_fh takes a dentry and creates a filehandle fragment which can implementation creates a filehandle fragment that encodes a 32bit inode
later be used to find/create a dentry for the same object. and generation number for the inode encoded, and if necessary the
same information for the parent.
decode_fh will probably make use of "find_exported_dentry". fh_to_dentry (mandatory)
This function lives in the "exportfs" module which a filesystem does Given a filehandle fragment, this should find the implied object and
not need unless it is being exported. So rather that calling create a dentry for it (possibly with d_alloc_anon).
find_exported_dentry directly, each filesystem should call it through
the find_exported_dentry pointer in it's export_operations table.
This field is set correctly by the exporting agent (e.g. nfsd) when a
filesystem is exported, and before any export operations are called.
find_exported_dentry needs three support functions from the fh_to_parent (optional but strongly recommended)
filesystem: Given a filehandle fragment, this should find the parent of the
get_name. When given a parent dentry and a child dentry, this implied object and create a dentry for it (possibly with d_alloc_anon).
should find a name in the directory identified by the parent May fail if the filehandle fragment is too small.
dentry, which leads to the object identified by the child dentry.
If no get_name function is supplied, a default implementation is
provided which uses vfs_readdir to find potential names, and
matches inode numbers to find the correct match.
get_parent. When given a dentry for a directory, this should return get_parent (optional but strongly recommended)
a dentry for the parent. Quite possibly the parent dentry will When given a dentry for a directory, this should return a dentry for
have been allocated by d_alloc_anon. the parent. Quite possibly the parent dentry will have been allocated
The default get_parent function just returns an error so any by d_alloc_anon. The default get_parent function just returns an error
filehandle lookup that requires finding a parent will fail. so any filehandle lookup that requires finding a parent will fail.
->lookup("..") is *not* used as a default as it can leave ".." ->lookup("..") is *not* used as a default as it can leave ".." entries
entries in the dcache which are too messy to work with. in the dcache which are too messy to work with.
get_dentry. When given an opaque datum, this should find the get_name (optional)
implied object and create a dentry for it (possibly with When given a parent dentry and a child dentry, this should find a name
d_alloc_anon). in the directory identified by the parent dentry, which leads to the
The opaque datum is whatever is passed down by the decode_fh object identified by the child dentry. If no get_name function is
function, and is often simply a fragment of the filehandle supplied, a default implementation is provided which uses vfs_readdir
fragment. to find potential names, and matches inode numbers to find the correct
decode_fh passes two datums through find_exported_dentry. One that match.
should be used to identify the target object, and one that can be
used to identify the object's parent, should that be necessary.
The default get_dentry function assumes that the datum contains an
inode number and a generation number, and it attempts to get the
inode using "iget" and check it's validity by matching the
generation number. A filesystem should only depend on the default
if iget can safely be used this way.
If decode_fh and/or encode_fh are left as NULL, then default
implementations are used. These defaults are suitable for ext2 and
extremely similar filesystems (like ext3).
The default encode_fh creates a filehandle fragment from the inode
number and generation number of the target together with the inode
number and generation number of the parent (if the parent is
required).
The default decode_fh extract the target and parent datums from the
filehandle assuming the format used by the default encode_fh and
passed them to find_exported_dentry.
A filehandle fragment consists of an array of 1 or more 4byte words, A filehandle fragment consists of an array of 1 or more 4byte words,
@ -172,5 +145,3 @@ generated by encode_fh, in which case it will have been padded with
nuls. Rather, the encode_fh routine should choose a "type" which nuls. Rather, the encode_fh routine should choose a "type" which
indicates the decode_fh how much of the filehandle is valid, and how indicates the decode_fh how much of the filehandle is valid, and how
it should be interpreted. it should be interpreted.

41
fs/exportfs/expfs.c
View file

@ -1,4 +1,13 @@
/*
* Copyright (C) Neil Brown 2002
* Copyright (C) Christoph Hellwig 2007
*
* This file contains the code mapping from inodes to NFS file handles,
* and for mapping back from file handles to dentries.
*
* For details on why we do all the strange and hairy things in here
* take a look at Documentation/filesystems/Exporting.
*/
#include <linux/exportfs.h> #include <linux/exportfs.h>
#include <linux/fs.h> #include <linux/fs.h>
#include <linux/file.h> #include <linux/file.h>
@ -9,19 +18,19 @@
#define dprintk(fmt, args...) do{}while(0) #define dprintk(fmt, args...) do{}while(0)
static int get_name(struct dentry *dentry, char *name, static int get_name(struct vfsmount *mnt, struct dentry *dentry, char *name,
struct dentry *child); struct dentry *child);
static int exportfs_get_name(struct dentry *dir, char *name, static int exportfs_get_name(struct vfsmount *mnt, struct dentry *dir,
struct dentry *child) char *name, struct dentry *child)
{ {
const struct export_operations *nop = dir->d_sb->s_export_op; const struct export_operations *nop = dir->d_sb->s_export_op;
if (nop->get_name) if (nop->get_name)
return nop->get_name(dir, name, child); return nop->get_name(dir, name, child);
else else
return get_name(dir, name, child); return get_name(mnt, dir, name, child);
} }
/* /*
@ -85,7 +94,7 @@ find_disconnected_root(struct dentry *dentry)
* It may already be, as the flag isn't always updated when connection happens. * It may already be, as the flag isn't always updated when connection happens.
*/ */
static int static int
reconnect_path(struct super_block *sb, struct dentry *target_dir) reconnect_path(struct vfsmount *mnt, struct dentry *target_dir)
{ {
char nbuf[NAME_MAX+1]; char nbuf[NAME_MAX+1];
int noprogress = 0; int noprogress = 0;
@ -108,7 +117,7 @@ reconnect_path(struct super_block *sb, struct dentry *target_dir)
pd->d_flags &= ~DCACHE_DISCONNECTED; pd->d_flags &= ~DCACHE_DISCONNECTED;
spin_unlock(&pd->d_lock); spin_unlock(&pd->d_lock);
noprogress = 0; noprogress = 0;
} else if (pd == sb->s_root) { } else if (pd == mnt->mnt_sb->s_root) {
printk(KERN_ERR "export: Eeek filesystem root is not connected, impossible\n"); printk(KERN_ERR "export: Eeek filesystem root is not connected, impossible\n");
spin_lock(&pd->d_lock); spin_lock(&pd->d_lock);
pd->d_flags &= ~DCACHE_DISCONNECTED; pd->d_flags &= ~DCACHE_DISCONNECTED;
@ -134,8 +143,8 @@ reconnect_path(struct super_block *sb, struct dentry *target_dir)
struct dentry *npd; struct dentry *npd;
mutex_lock(&pd->d_inode->i_mutex); mutex_lock(&pd->d_inode->i_mutex);
if (sb->s_export_op->get_parent) if (mnt->mnt_sb->s_export_op->get_parent)
ppd = sb->s_export_op->get_parent(pd); ppd = mnt->mnt_sb->s_export_op->get_parent(pd);
mutex_unlock(&pd->d_inode->i_mutex); mutex_unlock(&pd->d_inode->i_mutex);
if (IS_ERR(ppd)) { if (IS_ERR(ppd)) {
@ -148,7 +157,7 @@ reconnect_path(struct super_block *sb, struct dentry *target_dir)
dprintk("%s: find name of %lu in %lu\n", __FUNCTION__, dprintk("%s: find name of %lu in %lu\n", __FUNCTION__,
pd->d_inode->i_ino, ppd->d_inode->i_ino); pd->d_inode->i_ino, ppd->d_inode->i_ino);
err = exportfs_get_name(ppd, nbuf, pd); err = exportfs_get_name(mnt, ppd, nbuf, pd);
if (err) { if (err) {
dput(ppd); dput(ppd);
dput(pd); dput(pd);
@ -238,8 +247,8 @@ static int filldir_one(void * __buf, const char * name, int len,
* calls readdir on the parent until it finds an entry with * calls readdir on the parent until it finds an entry with
* the same inode number as the child, and returns that. * the same inode number as the child, and returns that.
*/ */
static int get_name(struct dentry *dentry, char *name, static int get_name(struct vfsmount *mnt, struct dentry *dentry,
struct dentry *child) char *name, struct dentry *child)
{ {
struct inode *dir = dentry->d_inode; struct inode *dir = dentry->d_inode;
int error; int error;
@ -255,7 +264,7 @@ static int get_name(struct dentry *dentry, char *name,
/* /*
* Open the directory ... * Open the directory ...
*/ */
file = dentry_open(dget(dentry), NULL, O_RDONLY); file = dentry_open(dget(dentry), mntget(mnt), O_RDONLY);
error = PTR_ERR(file); error = PTR_ERR(file);
if (IS_ERR(file)) if (IS_ERR(file))
goto out; goto out;
@ -372,7 +381,7 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
* filesystem root. * filesystem root.
*/ */
if (result->d_flags & DCACHE_DISCONNECTED) { if (result->d_flags & DCACHE_DISCONNECTED) {
err = reconnect_path(mnt->mnt_sb, result); err = reconnect_path(mnt, result);
if (err) if (err)
goto err_result; goto err_result;
} }
@ -424,7 +433,7 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
* connected to the filesystem root. The VFS really doesn't * connected to the filesystem root. The VFS really doesn't
* like disconnected directories.. * like disconnected directories..
*/ */
err = reconnect_path(mnt->mnt_sb, target_dir); err = reconnect_path(mnt, target_dir);
if (err) { if (err) {
dput(target_dir); dput(target_dir);
goto err_result; goto err_result;
@ -435,7 +444,7 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
* dentry for the inode we're after, make sure that our * dentry for the inode we're after, make sure that our
* inode is actually connected to the parent. * inode is actually connected to the parent.
*/ */
err = exportfs_get_name(target_dir, nbuf, result); err = exportfs_get_name(mnt, target_dir, nbuf, result);
if (!err) { if (!err) {
mutex_lock(&target_dir->d_inode->i_mutex); mutex_lock(&target_dir->d_inode->i_mutex);
nresult = lookup_one_len(nbuf, target_dir, nresult = lookup_one_len(nbuf, target_dir,

26
include/linux/exportfs.h
View file

@ -55,30 +55,8 @@ struct fid {
* @get_parent: find the parent of a given directory * @get_parent: find the parent of a given directory
* @get_dentry: find a dentry for the inode given a file handle sub-fragment * @get_dentry: find a dentry for the inode given a file handle sub-fragment
* *
* Description: * See Documentation/filesystems/Exporting for details on how to use
* The export_operations structure provides a means for nfsd to communicate * this interface correctly.
* with a particular exported file system - particularly enabling nfsd and
* the filesystem to co-operate when dealing with file handles.
*
* export_operations contains two basic operation for dealing with file
* handles, decode_fh() and encode_fh(), and allows for some other
* operations to be defined which standard helper routines use to get
* specific information from the filesystem.
*
* nfsd encodes information use to determine which filesystem a filehandle
* applies to in the initial part of the file handle. The remainder, termed
* a file handle fragment, is controlled completely by the filesystem. The
* standard helper routines assume that this fragment will contain one or
* two sub-fragments, one which identifies the file, and one which may be
* used to identify the (a) directory containing the file.
*
* In some situations, nfsd needs to get a dentry which is connected into a
* specific part of the file tree. To allow for this, it passes the
* function acceptable() together with a @context which can be used to see
* if the dentry is acceptable. As there can be multiple dentrys for a
* given file, the filesystem should check each one for acceptability before
* looking for the next. As soon as an acceptable one is found, it should
* be returned.
* *
* encode_fh: * encode_fh:
* @encode_fh should store in the file handle fragment @fh (using at most * @encode_fh should store in the file handle fragment @fh (using at most