Hi!
I've been looking at the possibility of cleanly implementing virtual
files (podfuk/avfs style) with the multiple mount thing in 2.4
kernels. Here is how it would work:
1) There needs to be a global lookup hook in VFS. If a real lookup
fails, the global lookup is invoked. The cached lookup path is not
effected.
2) The global lookup checks whether the name refers to a virtual file
(contains the magic char exists in /overlay). If it does, then the
file/dir from overlay is mounted on top of the negative dentry. The
dentry is not filled in, so the virtual file remains invisible.
For 1) the VFS needs to be modified, but with infinitesimal
performance effect. For 2) a kernel module could be used.
Great!
I've prepared a kernel patch against 2.4.0-test4 (just for comments on
this, NOT for inclusion in the main kernel), and a kernel module. I've
tested it with avfs, but it should also work for podfuk.
It looks much cleaner than my previous approaches. There seems to be
something strange with usage counts:
pavel@bug:/tmp/cz1_mbrola.tgz#utar$ ls -al
total 88
drwxr-xr-x 2 pavelusers3586 Mar 26 18:43 ./
drwxrwxrwt 38 root root81920 Jul 24 09:57 ../
drwxr-xr-x 42949 pavelusers 0 Mar 25 11:16 czech/
pavel@bug:/tmp/cz1_mbrola.tgz#utar$
Problems:
- The filesystem will be littered with these loopback mounts. This
should be cleared upon unmount, and possibly when the dcache is
shrunk. There was a similar requirement for new autofs IIRC.
- Creation/removal of virtual files are not handled by this code.
Comments?
Yes. You seem to have damaged locking:
@@ -281,12 +285,18 @@
lock_kernel();
result = dir-i_op-lookup(dir, dentry);
unlock_kernel();
- if (result)
+ if (result) {
dput(dentry);
- else
+up(dir-i_sem);
+}
+ else {
+ up(dir-i_sem);
result = dentry;
+ /* for avfs */
+ if (!dentry-d_inode lookup_global)
+ lookup_global(nd, dentry);
+ }
}
- up(dir-i_sem);
In case of !dentry, up(dir-i_sem) is not executed at all.
I have some cleanups to nredir.c module, they are attached (please
apply). Ouch, and there's slight problem with caching:
You access vmlinux#gz, then load nredir module, then access vmlinux#gz
again. But failure is already cached and you are out of luck :-(.
Pavel
--
I'm [EMAIL PROTECTED] "In my country we have almost anarchy and I don't care."
Panos Katsaloulis describing me w.r.t. patents at [EMAIL PROTECTED]
--- /tmp/nredir.c Mon Jul 24 10:03:48 2000
+++ nredir.cMon Jul 24 09:53:05 2000
@@ -1,4 +1,5 @@
#include linux/module.h
+#include linux/init.h
#include linux/kernel.h
#include linux/fs.h
#include linux/slab.h
@@ -7,7 +8,7 @@
#define NREDIR_VERSION "0.1"
-#define AVFS_MAGIC_CHAR '@'
+#define AVFS_MAGIC_CHAR '#'
#define OVERLAY_DIR "/overlay"
#define OVERLAY_DIR_LEN 8
@@ -99,7 +100,7 @@
return dentry;
}
-int init_module(void)
+static int __init init_nredir(void)
{
printk(KERN_INFO "nredir init (version %s)\n", NREDIR_VERSION);
@@ -111,11 +112,14 @@
}
-void cleanup_module(void)
+static void __exit cleanup_nredir(void)
{
printk(KERN_INFO "nredir cleanup\n");
-lookup_global = 0;
+lookup_global = NULL;
printk("lookup_global: %x\n", (int) lookup_global);
}
+
+module_init(init_nredir);
+module_exit(cleanup_nredir);
