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exportfs: update Exporting documentation

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Minor documentation updates:
	- refer to d_obtain_alias rather than d_alloc_anon
	- explain when to use d_splice_alias and when
	  d_materialise_unique.
	- cut some details of d_splice_alias/d_materialise_unique
	  implementation.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
J. Bruce Fields 2014-02-18 12:31:31 -05:00 committed by Al Viro
parent 8d80d7dabe
commit 9635389534
1 changed files with 23 additions and 15 deletions
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Documentation/filesystems/nfs/Exporting
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@ -66,23 +66,31 @@ b/ A per-superblock list "s_anon" of dentries which are the roots of
c/ Helper routines to allocate anonymous dentries, and to help attach c/ Helper routines to allocate anonymous dentries, and to help attach
loose directory dentries at lookup time. They are: loose directory dentries at lookup time. They are:
d_alloc_anon(inode) will return a dentry for the given inode. d_obtain_alias(inode) will return a dentry for the given inode.
If the inode already has a dentry, one of those is returned. If the inode already has a dentry, one of those is returned.
If it doesn't, a new anonymous (IS_ROOT and If it doesn't, a new anonymous (IS_ROOT and
DCACHE_DISCONNECTED) dentry is allocated and attached. DCACHE_DISCONNECTED) dentry is allocated and attached.
In the case of a directory, care is taken that only one dentry In the case of a directory, care is taken that only one dentry
can ever be attached. can ever be attached.
d_splice_alias(inode, dentry) will make sure that there is a d_splice_alias(inode, dentry) or d_materialise_unique(dentry, inode)
dentry with the same name and parent as the given dentry, and will introduce a new dentry into the tree; either the passed-in
which refers to the given inode. dentry or a preexisting alias for the given inode (such as an
If the inode is a directory and already has a dentry, then that anonymous one created by d_obtain_alias), if appropriate. The two
dentry is d_moved over the given dentry. functions differ in their handling of directories with preexisting
If the passed dentry gets attached, care is taken that this is aliases:
mutually exclusive to a d_alloc_anon operation. d_splice_alias will use any existing IS_ROOT dentry, but it will
If the passed dentry is used, NULL is returned, else the used return -EIO rather than try to move a dentry with a different
dentry is returned. This corresponds to the calling pattern of parent. This is appropriate for local filesystems, which
->lookup. should never see such an alias unless the filesystem is
corrupted somehow (for example, if two on-disk directory
entries refer to the same directory.)
d_materialise_unique will attempt to move any dentry. This is
appropriate for distributed filesystems, where finding a
directory other than where we last cached it may be a normal
consequence of concurrent operations on other hosts.
Both functions return NULL when the passed-in dentry is used,
following the calling convention of ->lookup.
Filesystem Issues Filesystem Issues
----------------- -----------------
@ -120,12 +128,12 @@ struct which has the following members:
fh_to_dentry (mandatory) fh_to_dentry (mandatory)
Given a filehandle fragment, this should find the implied object and Given a filehandle fragment, this should find the implied object and
create a dentry for it (possibly with d_alloc_anon). create a dentry for it (possibly with d_obtain_alias).
fh_to_parent (optional but strongly recommended) fh_to_parent (optional but strongly recommended)
Given a filehandle fragment, this should find the parent of the Given a filehandle fragment, this should find the parent of the
implied object and create a dentry for it (possibly with d_alloc_anon). implied object and create a dentry for it (possibly with
May fail if the filehandle fragment is too small. d_obtain_alias). May fail if the filehandle fragment is too small.
get_parent (optional but strongly recommended) get_parent (optional but strongly recommended)
When given a dentry for a directory, this should return a dentry for When given a dentry for a directory, this should return a dentry for