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From: Miklos Szeredi on 6 Jul 2010 13:50
In the old times pseudo-filesystems set the name of theroot dentry to
some prefix like "pipe:" and the name of the child dentry to "[123]"
and relied on a hack in __d_path() to replace the preceding slash with
the root's name to get "pipe:[123]".

Then the d_dname() dentry operation was introduced which solved the
same problem without having to pre-fill the name in each dentry.

Currently the following pseudo filesystems exist in the kernel:

perfmon
mtd
anon_inode
bdev
pipe
socket

Of these only perfmon, anon_inode, pipe and socket create
sub-dentries, all of which have now been switched to using d_dname().

bdev and mtd only create inodes.

This means that now the hack to overwrite the slash can be removed, so
for unreachable paths (e.g. within a detached mount) the path string
won't be polluted with garbage. For these cases a subsequent patch
will add a prefix, indicating that the path is unreachable.

Signed-off-by: Miklos Szeredi <mszeredi(a)suse.cz>
---
fs/dcache.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)

Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c 2010-07-06 18:08:06.000000000 +0200
+++ linux-2.6/fs/dcache.c 2010-07-06 18:08:12.000000000 +0200
@@ -1968,9 +1968,15 @@ out:
return retval;

global_root:
- retval += 1; /* hit the slash */
- if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
- goto Elong;
+ /*
+ * Filesystems needing to implement special "root names"
+ * should do so with ->d_dname()
+ */
+ if (IS_ROOT(dentry) &&
+ (dentry->d_name.len != 1 || dentry->d_name.name[0] != '/')) {
+ WARN(1, "Root dentry has weird name <%.*s>\n",
+ (int) dentry->d_name.len, dentry->d_name.name);
+ }
root->mnt = vfsmnt;
root->dentry = dentry;
goto out;

--
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Next: [PATCH 1/7] cachefiles: use path_get instead of lone dget