How to avoid rebooting Linux NFS-client when NFS-server is not available?

How to avoid rebooting Linux NFS-client when NFS-server is not available?

[Peter Funk]

Hello all,

We've researched this question for quite a while now and nobody here
found a solution to the following problem:

 1: A Linux computer is NFS client of some other Linux NFS server
    and has some active mounts and some processes working with files 
    on that NFS server.  

 2: Now the NFS server becomes unavailable and a system administrator 
    wants to clean up the situation on the NFS client computer without 
    having to reboot this client computer.

Is this possible?  And if how exactly?

Best Regards and many thanks in advance, 
Peter Funk
P.S.: umount -f -l did not work  
      System hangs for a long time in shutdown and shutdown 
      only succeeds without hard reset after reconnecting the
      NFS server.
-- 
Peter Funk, home: ✉Oldenburger Str.86, D-27777 Ganderkesee
mobile:+49-179-640-8878 phone:+49-421-20419-0 <http://www.artcom-gmbh.de/>
office: ArtCom GmbH, ✉Haferwende 2, D-28357 Bremen, Germany

Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?

[Jeff Layton]

On Wed, 24 Jul 2013 11:18:58 +0200
Peter Funk <pf@artcom-gmbh.de> wrote:

> Hello all,
> 
> We've researched this question for quite a while now and nobody here
> found a solution to the following problem:
> 
>  1: A Linux computer is NFS client of some other Linux NFS server
>     and has some active mounts and some processes working with files 
>     on that NFS server.  
> 
>  2: Now the NFS server becomes unavailable and a system administrator 
>     wants to clean up the situation on the NFS client computer without 
>     having to reboot this client computer.
> 
> Is this possible?  And if how exactly?
> 
> Best Regards and many thanks in advance, 
> Peter Funk
> P.S.: umount -f -l did not work  
>       System hangs for a long time in shutdown and shutdown 
>       only succeeds without hard reset after reconnecting the
>       NFS server.

The problem is likely that the lookup phase in the umount() syscall is
trying to revalidate the root of the mount. Since that server is down,
it's getting stuck.

Does this patch help at all? I'm hoping to get this into 3.12, and some
extra confirmation that it works would be helpful. It mentions about
the mount being stale, but it may also help the situation where it's
unavailable:

-----------------------[snip]-------------------------------

[PATCH] vfs: allow umount to handle mountpoints without revalidating them

Christopher reported a regression where he was unable to unmount a NFS
filesystem where the root had gone stale. The problem is that
d_revalidate handles the root of the filesystem differently from other
dentries, but d_weak_revalidate does not. We could simply fix this by
making d_weak_revalidate return success on IS_ROOT dentries, but there
are cases where we do want to revalidate the root of the fs.

A umount is really a special case. We generally aren't interested in
anything but the dentry and vfsmount that's attached at that point. If
the inode turns out to be stale we just don't care since the intent is
to stop using it anyway.

Try to handle this situation better by treating umount as a special
case in the lookup code. Have it resolve the parent using normal
means, and then do a lookup of the final dentry without revalidating
it. In most cases, the final lookup will come out of the dcache, but
the case where there's a trailing symlink or !LAST_NORM entry on the
end complicates things a bit.

Cc: Neil Brown <neilb@suse.de>
Reported-by: Christopher T Vogan <cvogan@us.ibm.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 fs/namei.c            | 182 ++++++++++++++++++++++++++++++++++++++++++++++++++
 fs/namespace.c        |   2 +-
 include/linux/namei.h |   1 +
 3 files changed, 184 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 8b61d10..d9f65bd 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2184,6 +2184,188 @@ user_path_parent(int dfd, const char __user *path, struct nameidata *nd,
 	return s;
 }
 
+/**
+ * umount_lookup_last - look up last component for umount
+ * @nd:   pathwalk nameidata - currently pointing at parent directory of "last"
+ * @path: pointer to container for result
+ *
+ * This is a special lookup_last function just for umount. In this case, we
+ * need to resolve the path without doing any revalidation.
+ *
+ * The nameidata should be the result of doing a LOOKUP_PARENT pathwalk. Since
+ * mountpoints are always pinned in the dcache, their ancestors are too. Thus,
+ * in almost all cases, this lookup will be served out of the dcache. The only
+ * cases where it won't are if nd->last refers to a symlink or the path is
+ * bogus and it doesn't exist.
+ *
+ * Returns:
+ * -error: if there was an error during lookup. This includes -ENOENT if the
+ *         lookup found a negative dentry. The nd->path reference will also be
+ *         put in this case.
+ *
+ * 0:      if we successfully resolved nd->path and found it to not to be a
+ *         symlink that needs to be followed. "path" will also be populated.
+ *         The nd->path reference will also be put.
+ *
+ * 1:      if we successfully resolved nd->last and found it to be a symlink
+ *         that needs to be followed. "path" will be populated with the path
+ *         to the link, and nd->path will *not* be put.
+ */
+static int
+umount_lookup_last(struct nameidata *nd, struct path *path)
+{
+	int error = 0;
+	struct dentry *dentry;
+	struct dentry *dir = nd->path.dentry;
+
+	if (unlikely(nd->flags & LOOKUP_RCU)) {
+		WARN_ON_ONCE(1);
+		error = -ECHILD;
+		goto error_check;
+	}
+
+	nd->flags &= ~LOOKUP_PARENT;
+
+	if (unlikely(nd->last_type != LAST_NORM)) {
+		error = handle_dots(nd, nd->last_type);
+		if (!error)
+			dentry = dget(nd->path.dentry);
+		goto error_check;
+	}
+
+	mutex_lock(&dir->d_inode->i_mutex);
+	dentry = d_lookup(dir, &nd->last);
+	if (!dentry) {
+		/*
+		 * No cached dentry. Mounted dentries are pinned in the cache,
+		 * so that means that this dentry is probably a symlink or the
+		 * path doesn't actually point to a mounted dentry.
+		 */
+		dentry = d_alloc(dir, &nd->last);
+		if (!dentry) {
+			error = -ENOMEM;
+		} else {
+			dentry = lookup_real(dir->d_inode, dentry, nd->flags);
+			if (IS_ERR(dentry))
+				error = PTR_ERR(dentry);
+		}
+	}
+	mutex_unlock(&dir->d_inode->i_mutex);
+
+error_check:
+	if (!error) {
+		if (!dentry->d_inode) {
+			error = -ENOENT;
+			dput(dentry);
+		} else {
+			path->dentry = dentry;
+			path->mnt = mntget(nd->path.mnt);
+			if (should_follow_link(dentry->d_inode,
+						nd->flags & LOOKUP_FOLLOW))
+				return 1;
+			follow_mount(path);
+		}
+	}
+	terminate_walk(nd);
+	return error;
+}
+
+/**
+ * path_umountat - look up a path to be umounted
+ * @dfd:	directory file descriptor to start walk from
+ * @name:	full pathname to walk
+ * @flags:	lookup flags
+ * @nd:		pathwalk nameidata
+ *
+ * Look up the given name, but don't attempt to revalidate the last component.
+ * Returns 0 and "path" will be valid on success; Retuns error otherwise.
+ */
+static int
+path_umountat(int dfd, const char *name, struct path *path, unsigned int flags)
+{
+	struct file *base = NULL;
+	struct nameidata nd;
+	int err;
+
+	err = path_init(dfd, name, flags | LOOKUP_PARENT, &nd, &base);
+	if (unlikely(err))
+		return err;
+
+	current->total_link_count = 0;
+	err = link_path_walk(name, &nd);
+	if (err)
+		goto out;
+
+	/* If we're in rcuwalk, drop out of it to handle last component */
+	if (nd.flags & LOOKUP_RCU) {
+		err = unlazy_walk(&nd, NULL);
+		if (err) {
+			terminate_walk(&nd);
+			goto out;
+		}
+	}
+
+	err = umount_lookup_last(&nd, path);
+	while (err > 0) {
+		void *cookie;
+		struct path link = *path;
+		err = may_follow_link(&link, &nd);
+		if (unlikely(err))
+			break;
+		nd.flags |= LOOKUP_PARENT;
+		err = follow_link(&link, &nd, &cookie);
+		if (err)
+			break;
+		err = umount_lookup_last(&nd, path);
+		put_link(&nd, &link, cookie);
+	}
+out:
+	if (base)
+		fput(base);
+
+	if (nd.root.mnt && !(nd.flags & LOOKUP_ROOT))
+		path_put(&nd.root);
+
+	return err;
+}
+
+/**
+ * user_path_umountat - lookup a path from userland in order to umount it
+ * @dfd:	directory file descriptor
+ * @name:	pathname from userland
+ * @flags:	lookup flags
+ * @path:	pointer to container to hold result
+ *
+ * A umount is a special case for path walking. We're not actually interested
+ * in the inode in this situation, and ESTALE errors can be a problem. We
+ * simply want track down the dentry and vfsmount attached at the mountpoint
+ * and avoid revalidating the last component.
+ *
+ * Returns 0 and populates "path" on success.
+ */
+int
+user_path_umountat(int dfd, const char __user *name, unsigned int flags,
+			struct path *path)
+{
+	struct filename *s = getname(name);
+	int error;
+
+	if (IS_ERR(s))
+		return PTR_ERR(s);
+
+	error = path_umountat(dfd, s->name, path, flags | LOOKUP_RCU);
+	if (unlikely(error == -ECHILD))
+		error = path_umountat(dfd, s->name, path, flags);
+	if (unlikely(error == -ESTALE))
+		error = path_umountat(dfd, s->name, path, flags | LOOKUP_REVAL);
+
+	if (likely(!error))
+		audit_inode(s, path->dentry, 0);
+
+	putname(s);
+	return error;
+}
+
 /*
  * It's inline, so penalty for filesystems that don't use sticky bit is
  * minimal.
diff --git a/fs/namespace.c b/fs/namespace.c
index 7b1ca9b..5d2676a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1318,7 +1318,7 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
 	if (!(flags & UMOUNT_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
 
-	retval = user_path_at(AT_FDCWD, name, lookup_flags, &path);
+	retval = user_path_umountat(AT_FDCWD, name, lookup_flags, &path);
 	if (retval)
 		goto out;
 	mnt = real_mount(path.mnt);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 5a5ff57..cd09751 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -58,6 +58,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 
 extern int user_path_at(int, const char __user *, unsigned, struct path *);
 extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);
+extern int user_path_umountat(int, const char __user *, unsigned int, struct path *);
 
 #define user_path(name, path) user_path_at(AT_FDCWD, name, LOOKUP_FOLLOW, path)
 #define user_lpath(name, path) user_path_at(AT_FDCWD, name, 0, path)
-- 
1.8.3.1

Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?

[Dick Streefland, rnews]

Peter Funk <pf@artcom-gmbh.de> wrote:
| We've researched this question for quite a while now and nobody here
| found a solution to the following problem:
| 
|  1: A Linux computer is NFS client of some other Linux NFS server
|     and has some active mounts and some processes working with files 
|     on that NFS server.  
| 
|  2: Now the NFS server becomes unavailable and a system administrator 
|     wants to clean up the situation on the NFS client computer without 
|     having to reboot this client computer.
| 
| Is this possible?  And if how exactly?

What you could try is temporarily add the IP number of the dead NFS
server to another NFS server. The other NFS server should reject any
request for the dead mount, and the client can continue with an error.

-- 
Dick

Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?

[Peter Funk]

Dick Streefland wrote 24.07.2013 13:03:
> Peter Funk <pf@artcom-gmbh.de> wrote:
> | We've researched this question for quite a while now and nobody here
> | found a solution to the following problem:
> | 
> |  1: A Linux computer is NFS client of some other Linux NFS server
> |     and has some active mounts and some processes working with files 
> |     on that NFS server.  
> | 
> |  2: Now the NFS server becomes unavailable and a system administrator 
> |     wants to clean up the situation on the NFS client computer without 
> |     having to reboot this client computer.
> | 
> | Is this possible?  And if how exactly?
> 
> What you could try is temporarily add the IP number of the dead NFS
> server to another NFS server. The other NFS server should reject any
> request for the dead mount, and the client can continue with an error.

Indeed: This workaround seems to work!

Assume example: The NFS-server has IP 192.168.123.45 and the client
has also the nfs-kernel-server package installed and it is running.
Then this sequence on the client did the trick::

    ifconfig eth0:fakesrv 192.168.123.45 up
    umount -f -l ....
    umount -f -l ....
    ....
    ifconfig eth0:fakesrv down

Best Regards and many thanks for your suggestion,
Peter Funk
-- 
Peter Funk, home: ✉Oldenburger Str.86, D-27777 Ganderkesee
mobile:+49-179-640-8878 phone:+49-421-20419-0 <http://www.artcom-gmbh.de/>
office: ArtCom GmbH, ✉Haferwende 2, D-28357 Bremen, Germany

Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?

[Michael Richardson]

Peter Funk <pf@artcom-gmbh.de> wrote:
    > Indeed: This workaround seems to work!

    > Assume example: The NFS-server has IP 192.168.123.45 and the client
    > has also the nfs-kernel-server package installed and it is running.
    > Then this sequence on the client did the trick::

    > ifconfig eth0:fakesrv 192.168.123.45 up
    > umount -f -l ....
    > umount -f -l ....
    > ....
    > ifconfig eth0:fakesrv down

Good idea, but I suggest doing it on lo rather than eth0.

Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?

[devzero]

>Indeed: This workaround seems to work!

Unfortunately not in every situation.

We have differnt XEN servers (Citrix XS7) at remote location which have hung/stale NFS mount problems at regular intervals (.ISO storage repo is mounted via WAN) and i always need to reboot, which really really(!) sucks.

At least a fake NFS server as described below releases the stuck mount, i.e. df -h and other processes touching do not hang anymore, so at least this workaround helps to some degree... 

BUT:

# umount /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e
umount.nfs: /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e: Stale file handle

# mount|grep xen-sr-iso
172.16.28.10:/mnt/S2V2/xen-sr-iso on /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e type nfs (rw,relatime,vers=3,rsize=131072,wsize=131072,namlen=255,acdirmin=0,acdirmax=0,soft,proto=tcp,timeo=600,retrans=2147483647,sec=sys,mountaddr=172.16.28.10,mountvers=3,mountport=680,mountproto=tcp,local_lock=none,addr=172.16.28.10)

# umount -l -f /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e
umount.nfs: /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e: Stale file handle

# mount|grep xen-sr-iso
172.16.28.10:/mnt/S2V2/xen-sr-iso on /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e type nfs (rw,relatime,vers=3,rsize=131072,wsize=131072,namlen=255,acdirmin=0,acdirmax=0,soft,proto=tcp,timeo=600,retrans=2147483647,sec=sys,mountaddr=172.16.28.10,mountvers=3,mountport=680,mountproto=tcp,local_lock=none,addr=172.16.28.10)

# ls -la
ls: cannot access 2b5c5c60-3744-c860-28a7-eb106d3a339e: Stale file handle
total 0
drwx------  3 root root   60 Feb 18 17:43 .
drwxr-xr-x 36 root root 1660 Jun  7 18:45 ..
d?????????  ? ?    ?       ?            ? 2b5c5c60-3744-c860-28a7-eb106d3a339e

Any hint on how to circumvent rebooting to remount the nfs share or proactively avoid stale NFS mounts would be very appreciated. (disabling NFS by module unload/load is no option, as our XEN servers do have other NFS mounts for shared storage)

regards
Roland

ps:
I`m not sure if linux-nfs ML will allow anonymous posts (probably not), so maybe someone subscribed be so kind to reply with list cc´ed. I`d like to avoid subscribing to a list because of a single post...




>List:       linux-nfs
>Subject:    Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?
>From:       Peter Funk <pf () artcom-gmbh ! de>
>Date:       2013-07-26 12:08:49
>Message-ID: 20130726120849.GA12584 () pfmaster
>[Download message RAW]

>Dick Streefland wrote 24.07.2013 13:03:
>> Peter Funk <pf@artcom-gmbh.de> wrote:
>> | We've researched this question for quite a while now and nobody here
>> | found a solution to the following problem:
>> | 
>> |  1: A Linux computer is NFS client of some other Linux NFS server
>> |     and has some active mounts and some processes working with files 
>> |     on that NFS server.  
>> | 
>> |  2: Now the NFS server becomes unavailable and a system administrator 
>> |     wants to clean up the situation on the NFS client computer without 
>> |     having to reboot this client computer.
>> | 
>> | Is this possible?  And if how exactly?
>> 
>> What you could try is temporarily add the IP number of the dead NFS
>> server to another NFS server. The other NFS server should reject any
>> request for the dead mount, and the client can continue with an error.
>
>Indeed: This workaround seems to work!
>
>Assume example: The NFS-server has IP 192.168.123.45 and the client
>has also the nfs-kernel-server package installed and it is running.
>Then this sequence on the client did the trick::
>
>    ifconfig eth0:fakesrv 192.168.123.45 up
>    umount -f -l ....
>    umount -f -l ....
>    ....
>    ifconfig eth0:fakesrv down
>
>Best Regards and many thanks for your suggestion,
>Peter Funk

Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?

[J. Bruce Fields]

On Thu, Jun 08, 2017 at 10:49:50AM +0200, devzero@web.de wrote:
> Any hint on how to circumvent rebooting to remount the nfs share or proactively avoid stale NFS mounts would be very appreciated. (disabling NFS by module unload/load is no option, as our XEN servers do have other NFS mounts for shared storage)

Neil had some recent posts that might be relevant:

	http://marc.info/?l=util-linux-ng&m=149662998931157&w=2

Ripping out the storage from underneath your applications is a pretty
drastic step and may lose data, so NFS hasn't traditionally tried very
hard to make it easy.  But it may be possible at this point if you kill
-9 all the users and umount carefully.

> I`m not sure if linux-nfs ML will allow anonymous posts (probably
> not), so maybe someone subscribed be so kind to reply with list cc´ed.
> I`d like to avoid subscribing to a list because of a single post...

That's fine.

--b.

Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 4742 bytes --]

On Thu, Jun 08 2017, devzero@web.de wrote:

>>Indeed: This workaround seems to work!
>
> Unfortunately not in every situation.
>
> We have differnt XEN servers (Citrix XS7) at remote location which have hung/stale NFS mount problems at regular intervals (.ISO storage repo is mounted via WAN) and i always need to reboot, which really really(!) sucks.
>
> At least a fake NFS server as described below releases the stuck mount, i.e. df -h and other processes touching do not hang anymore, so at least this workaround helps to some degree... 
>
> BUT:
>
> # umount /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e
> umount.nfs: /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e: Stale file handle
>
> # mount|grep xen-sr-iso
> 172.16.28.10:/mnt/S2V2/xen-sr-iso on /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e type nfs (rw,relatime,vers=3,rsize=131072,wsize=131072,namlen=255,acdirmin=0,acdirmax=0,soft,proto=tcp,timeo=600,retrans=2147483647,sec=sys,mountaddr=172.16.28.10,mountvers=3,mountport=680,mountproto=tcp,local_lock=none,addr=172.16.28.10)
>
> # umount -l -f /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e
> umount.nfs: /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e: Stale file handle

This really shouldn't happen.
Since Linux 3.12 (commit 8033426e6bdb) it has been possible to
umount an NFS filesystem, no matter what state it is in.

Since util-linux 2.23 (commit 6d5d2b5fd342308), umount -l or umount -f
should have avoided statting the filesystem.

If you have a new util-linux (mount -V) and new kernel (uname -a),
then I'd be interested to see
  strace -o /tmp/strace -f umount -f /run/......



>
> # mount|grep xen-sr-iso
> 172.16.28.10:/mnt/S2V2/xen-sr-iso on /run/sr-mount/2b5c5c60-3744-c860-28a7-eb106d3a339e type nfs (rw,relatime,vers=3,rsize=131072,wsize=131072,namlen=255,acdirmin=0,acdirmax=0,soft,proto=tcp,timeo=600,retrans=2147483647,sec=sys,mountaddr=172.16.28.10,mountvers=3,mountport=680,mountproto=tcp,local_lock=none,addr=172.16.28.10)
>
> # ls -la
> ls: cannot access 2b5c5c60-3744-c860-28a7-eb106d3a339e: Stale file handle
> total 0
> drwx------  3 root root   60 Feb 18 17:43 .
> drwxr-xr-x 36 root root 1660 Jun  7 18:45 ..
> d?????????  ? ?    ?       ?            ? 2b5c5c60-3744-c860-28a7-eb106d3a339e
>
> Any hint on how to circumvent rebooting to remount the nfs share or proactively avoid stale NFS mounts would be very appreciated. (disabling NFS by module unload/load is no option, as our XEN servers do have other NFS mounts for shared storage)
>
> regards
> Roland
>
> ps:
> I`m not sure if linux-nfs ML will allow anonymous posts (probably not), so maybe someone subscribed be so kind to reply with list cc´ed. I`d like to avoid subscribing to a list because of a single post...
>

We aren't that closed-minded ;-)  Your message went to the list.

NeilBrown

>
>
>
>>List:       linux-nfs
>>Subject:    Re: How to avoid rebooting Linux NFS-client when NFS-server is not available?
>>From:       Peter Funk <pf () artcom-gmbh ! de>
>>Date:       2013-07-26 12:08:49
>>Message-ID: 20130726120849.GA12584 () pfmaster
>>[Download message RAW]
>
>>Dick Streefland wrote 24.07.2013 13:03:
>>> Peter Funk <pf@artcom-gmbh.de> wrote:
>>> | We've researched this question for quite a while now and nobody here
>>> | found a solution to the following problem:
>>> | 
>>> |  1: A Linux computer is NFS client of some other Linux NFS server
>>> |     and has some active mounts and some processes working with files 
>>> |     on that NFS server.  
>>> | 
>>> |  2: Now the NFS server becomes unavailable and a system administrator 
>>> |     wants to clean up the situation on the NFS client computer without 
>>> |     having to reboot this client computer.
>>> | 
>>> | Is this possible?  And if how exactly?
>>> 
>>> What you could try is temporarily add the IP number of the dead NFS
>>> server to another NFS server. The other NFS server should reject any
>>> request for the dead mount, and the client can continue with an error.
>>
>>Indeed: This workaround seems to work!
>>
>>Assume example: The NFS-server has IP 192.168.123.45 and the client
>>has also the nfs-kernel-server package installed and it is running.
>>Then this sequence on the client did the trick::
>>
>>    ifconfig eth0:fakesrv 192.168.123.45 up
>>    umount -f -l ....
>>    umount -f -l ....
>>    ....
>>    ifconfig eth0:fakesrv down
>>
>>Best Regards and many thanks for your suggestion,
>>Peter Funk
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