Re: "Too many levels of symbolic links"

[Donald Buczek <buczek@molgen.mpg.de>]

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Am 28.02.2014 14:29, schrieb Alexander Viro:
> On Fri, Feb 28, 2014 at 01:12:58PM +0100, Donald Buczek wrote:
>
>> Obviously, "cleared mounted on dentry" is missing.
>>
>> It looks like we enter put_mountpoint() but don't get to
>> dentry->d_flags &= ~DCACHE_MOUNTED;
>>
>> mp->m_count is not zero probably.
>>
>> What does it mean? The mount is still locked but not in the mount hash?
> No, it means that something else is mounted on the same dentry (in another
> part of mount tree, obviously).
>
> If you mount the same fs on two different mountpoints, e.g.
> mount /dev/sda1 /mnt
> mount /dev/sda1 /tmp/foo
> you will have the same dentries seen in two places.  Now,
> mount /dev/sdb11 /mnt/a
> mount /dev/sdc5 /tmp/foo/a
>
> and you've got two different filesystems mounted on two different places
> (/mnt/a and /tmp/foo/a).  These two places have different vfsmounts,
> but the same dentry.  struct mountpoint is associated with dentry, so
> it's also the same for both.  And it serves as a mountpoint for two
> vfsmounts - one for fs from sdb11, another for fs from sdc5.
>
> Now umount /mnt/a; one of those two vfsmounts is gone now.  struct mountpoint
> survives, of course, and dentry is *still* a mountpoint.  sdc5 is still
> mounted on /tmp/foo/a, after all...

Thanks. So I guess, the idea of "struct mountpoint" is to make the 
dentries smaller by not having the mount count embedded in each one, 
99.9%  not needing it?

OMG, I've just found this in the log:

> 91286 2014-02-26T14:09:56.830515+01:00 kasslerbraten kernel: 
> [195977.007799] pid 8644: d_set_mounted: dentry=ffff88004690c710 root
> 91287 2014-02-26T14:09:56.830527+01:00 kasslerbraten kernel: 
> [195977.007802] pid 8644: d_set_mounted: set mounted on 
> dentry=ffff88004690c710 root
> 91288 2014-02-26T14:09:56.830529+01:00 kasslerbraten kernel: 
> [195977.007873] pid 8644: put_mountpoint: mp=ffff8800b8aa1a38
> 91289 2014-02-26T14:09:56.830530+01:00 kasslerbraten kernel: 
> [195977.007877] pid 8644: d_set_mounted: dentry=ffff8800ca45e810 tmp
> 91290 2014-02-26T14:09:56.830535+01:00 kasslerbraten kernel: 
> [195977.007878] pid 8644: d_set_mounted: set mounted on 
> dentry=ffff8800ca45e810 tmp
> 91291 2014-02-26T14:09:56.830536+01:00 kasslerbraten kernel: 
> [195977.007881] pid 8644: put_mountpoint: mp=ffff8800b8aa1a38
> 91292 2014-02-26T14:09:56.830537+01:00 kasslerbraten kernel: 
> [195977.007900] pid 8644: d_set_mounted: dentry=ffff880046960450 
> old-root-mjn70Q
> 91293 2014-02-26T14:09:56.830538+01:00 kasslerbraten kernel: 
> [195977.007901] pid 8644: d_set_mounted: set mounted on 
> dentry=ffff880046960450 old-root-mjn70Q
> 91294 2014-02-26T14:09:56.830539+01:00 kasslerbraten kernel: 
> [195977.007903] pid 8644: put_mountpoint: mp=ffff8800a5f1dbd0
> 91295 2014-02-26T14:09:56.830540+01:00 kasslerbraten kernel: 
> [195977.007904] pid 8644: put_mountpoint: cleared mounted on 
> dentry=ffff88004690c710 root
> 91296 2014-02-26T14:09:56.830541+01:00 kasslerbraten kernel: 
> [195977.007905] pid 8644: put_mountpoint: mp=ffff8800a5f1da90
> 91297 2014-02-26T14:09:56.830541+01:00 kasslerbraten kernel: 
> [195977.007954] pid 8644: put_mountpoint: mp=0000014f00490049
> 91298 2014-02-26T14:09:56.830542+01:00 kasslerbraten kernel: 
> [195977.007955] pid 8644: put_mountpoint: mp=0000000000000006
> 91299 2014-02-26T14:09:56.830543+01:00 kasslerbraten kernel: 
> [195977.007961] pid 8644: put_mountpoint: mp=ffff8800a5f1dbd0
> 91300 2014-02-26T14:09:56.830544+01:00 kasslerbraten kernel: 
> [195977.007963] pid 8644: put_mountpoint: mp=ffff8800a6023d10
> 91301 2014-02-26T14:09:56.830544+01:00 kasslerbraten kernel: 
> [195977.007963] pid 8644: put_mountpoint: mp=ffff8800a6023d10
> 91302 2014-02-26T14:09:56.830545+01:00 kasslerbraten kernel: 
> [195977.007964] pid 8644: put_mountpoint: mp=ffff8800a5f1dbd0

What is this? Where does "root" and "old-root-" come from? Why does 8644 
survive dereferencing mp=0000000000000006 from the kernel? Is this still 
related to autofs?

D.

-- 
Donald Buczek
buczek@molgen.mpg.de
Tel: +49 30 8413 1433



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