On 06/04/2014 11:21 PM, J. Bruce Fields wrote:
From: "J. Bruce Fields" <bfields@xxxxxxxxxx>
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@xxxxxxxxxx>
---
Documentation/filesystems/nfs/Exporting | 38 +++++++++++++++++++------------
1 file changed, 23 insertions(+), 15 deletions(-)
diff --git a/Documentation/filesystems/nfs/Exporting b/Documentation/filesystems/nfs/Exporting
index e543b1a..9b7de5b 100644
--- a/Documentation/filesystems/nfs/Exporting
+++ b/Documentation/filesystems/nfs/Exporting
@@ -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
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 it doesn't, a new anonymous (IS_ROOT and
DCACHE_DISCONNECTED) dentry is allocated and attached.
In the case of a directory, care is taken that only one dentry
can ever be attached.
- d_splice_alias(inode, dentry) will make sure that there is a
- dentry with the same name and parent as the given dentry, and
- which refers to the given inode.
- If the inode is a directory and already has a dentry, then that
- dentry is d_moved over the given dentry.
- If the passed dentry gets attached, care is taken that this is
- mutually exclusive to a d_alloc_anon operation.
- If the passed dentry is used, NULL is returned, else the used
- dentry is returned. This corresponds to the calling pattern of
- ->lookup.
-
+ d_splice_alias(inode, dentry) or d_materialise_unique(dentry, inode)
+ will introduce a new dentry into the tree; either the passed-in
+ dentry or a preexisting alias for the given inode (such as an
+ anonymous one created by d_obtain_alias), if appropriate. The two
+ functions differ in their handling of directories with preexisting
+ aliases:
+ d_splice_alias will use any existing IS_ROOT dentry, but it will
+ return -EIO rather than try to move a dentry with a different
+ parent. This is appropriate for local filesystems, which
+ 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_obtain_alias 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.
+
Hmm, is this really supposed to be in nfs/Exporting? Wouldn't be a new
chapter in vfs.txt or a new file dache.c more appropriate? I was looking
in the past for their documentation, but then thought it does not exist
- one needs to use these even without nfs exports.
Thanks,
Bernd
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