Re: [autofs] [RFC PATCH]autofs4: hang and proposed fix

["William H. Taber" <wtaber@us.ibm.com>]

Ian Kent wrote:
  >
> OK I think I'm starting to get what your saying. I guess I didn't want to 
> hear it because I've been moving toward pushing everything to revalidate.
> 
> For some reason I woke up early this morning. I read your reply and I've 
> been thinking about it all day (hard to change gears when there's a 
> challenge like this, bad for work, fun for me, I'm sure I'll get cained 
> for being somewhere else).
> 
> To verify I've got it this time all you are saying is, avoid revalidate by 
> keeping all unmounted dentrys unhashed? Nothing much more really?
> 
> I haven't really thought it through completely yet. You've noticed I'm 
> not a quick study no doubt.
Who am I to complain about that? :^)
> 
> So far it seems doable though. One problem point could be if one of these 
> lookups come in during user space daemon startup but that's detail atm.
> 
> Following startup I have two cases to deal with (i'm only listing them 
> cause I think you missed case 2):
> 
> 1) No directory exists - it's created by the daemon during the callback to 
> the userspace before it perform the mount.
> 
> 2) Directory already exists - created at startup before any mount 
> activity.
I am a little unclear about this.  Which directory are you talking 
about?  The mount-over directory or the directory being mounted?  My 
picture of things (and I haven't verified this by doing anything mundane 
like reading the code) is that There is a directory (such as /net ) 
which is of type autofs and as needed new directories of type autofs are 
created under it for the various hosts/filenames.  Over these 
subdirectories (the mount-over directories) get mounted the remote 
filesystems, usually of type NFS.  Is this a correct understanding?
> 
> Case 2 needs special attention. To achieve this I would need to have two 
> types of unhashed dentry, valid and invalid (perhaps because of a ENOENT 
> return from the daemon on mount).
> 
If my understanding above is correct, I don't think you need to hide the 
dentry for the autofs mount-over directory.  If there is an active 
mount, then the dentries d_mounted flag will be set and the normal 
mountpoint traversal will work.  If nothing is mounted here then the 
autofs mount-over directory lookup functions will be called.  This is 
where the actual mount request gets triggered.  The dentry created here 
should not be added to the dentry cache until the dentry is actually 
ready to use.  It has to be kept in a way that can be found by 
subsequent calls to lookup in case there are multiple requests for it. 
The trick is that the first lookup to succeed has to be the one for the 
mount request.  But once it is on the dentry hash chain, revalidate has 
to be careful because if the revalidate fails then the dentry will be 
invalidated.  And if revalidate succeeds then everything needs to be 
setup so that folow_down will work.  Hmm.  I will have to think about 
this some more.
> Not really that hard to do I think.
> 
> I'd need to rework the readdir code to fill unhashed, valid dentrys and I 
> can set status on the return codes from the daemon. There's likely a bunch 
> of other detail as well.
> 
> Unfortuneatly for me this is only half of the solution ... see below.
> 
> 
>>>>autofs4_revalidate cannot pend.  I have looked some more at the 
>>>
>>>
>>>Exactly and that's what I'm suggesting. Take account of what the
>>>lookup_one_len is advertised to do. My point is that lookup_one_len is not
>>>supposed to follow mounts or soft links, by definition, so it shouldn't
>>>cause autofs to trigger any mounts. If a filesystem wants to use it then it
>>>then it needs to take account of its defined behaviour, warts and all.
>>>
>>
>>I am not expecting lookup_one_len to follow mount points.  I expect to follow
>>them myself.  But I do expect that if this is a mountpoint, that autofs will
>>set it up for me.  If not, what is the point of an automounter?
>>
>>>>real_lookup code, and it is prepared for the case in which the lookup
>>>>function returns a dentry other than the one passed in.  So here is a
>>>>proposal that might work (but I haven't looked at the autofs4 code to
>>>>verify this.)
>>>>1) A lookup request is made for a non-existant automounted file.
>>>>Real_lookup calls autofs4_lookup.
>>>>2) Autofs4_lookup saves the information about this request somewhere it
>>>>can find it again and wakes up the automount demon that it has work to do.
>>>>It does not put a dentry in the dentry cache and it then releases the
>>>>parent i_sem and waits for the mount to complete.
>>>>3) Any subsequent lookup for this directory that is not from the automount
>>>>demon will look for a mount request in progress, and if found, it will
>>>>also release the parent lock and add itself to the wait queue.
>>>>4)The automount demon will run and get the information that it needs to
>>>>complete the mount request and  then issue the mount.  The lookup request
>>>>from mount will call real_lookup.  Since the demon is in OZ mode it does
>>>>not pend, it fills in the dentry and when the dentry is fully ready for
>>>>consumption, it calls d_add and wakes up the waiters.
>>>>5) When the waiters wake up, they get the new dentry and real_lookup will
>>>>discard the one that had been allocated.
>>>>
>>>>This keeps all of the waiting inside the autofs4 lookup function where the
>>>>lock state is defined.  I realize that this may be a lot of work, but I
>>>>haven't seen a possible solution that doesn't involve that.
>>>
>>>
>>>Sounds like a lot of work and likely quite interesting but directories can
>>>and often do exist in the autofs filesystem that don't have active mounts. 
>>>For these directories only the revaidate method is called at auto mount
>>>time. It's worth remembering that, as autofs is a pseudo filesystem, it pins
>>>the dentry for each of its objects so they don't go away. Maybe you're
>>>suggesting I change this?
>>
>>Exactly.  If the dentries are unhashed at umount time then the revalidate case
>>is not an issue.  I don't know the how much work is invovled in setting things
>>up in the first place so you might want to cache your unused dentries yourself
>>if that avoids having to reread the autofs configuration files.  But that is
>>an implementation detail for you to consider.
>>
>>>Sorry, I don't mean to be rude but I'm suggesting your using lookup_one_len
>>>incorectly. I'll need to look at other code to see if this actually holds
>>>true, but, as automounting is usually not the first thing that people think
>>>of when they are writting a filesystem I expect I won't get much support
>>>from there either. 
>>
>>I don't know what you mean by using it incorrectly.  We have a shadow
>>filesytem and our lookup function is being called and we are trying to find
>>the corresponding file/directory in the root filesystem.  We are calling
>>lookup_one_len because we are trying to find the next name in the path.  We
>>are prepared to handle mountpoint crossings, but as I said above, the
>>mountpoint needs to be setup so we can cross it.  We cannot call path_walk or
>>its variants because we do not have the entire path.
>>
>>
>>
>>>What I'm proposing is:
>>>
>>>1) lookup_one_len should never cause anything to be auto
>>>   mounted because of its defined behaviour and autofs
>>>   should behave in line with this definition.
>>
>>What defined behaviour?  The purpose of autofs is to automount directories.  I
>>am not looking for you to cross a mountpoint for me, I just want you to setup
>>the mount so I can cross it myself.
>>
>>>2) The filesystem that calls lookup_one_len directly or
>>>   indirectly is responsibe for checking if it has walked
>>>   onto an autofs dentry and decide what action it should
>>>   take.
>>
>>And what action would that be?  Not enter into any autofs directory tree?  Do
>>the mount myself?  Return ENOENT?  Return inconsistent results based on
>>whether someone else has triggered the automount for me?
> 
> 
> I was thinking of something like EAGAIN to the calling fs - meaning, ok 
> but I need to be revalidated (lockles) for you to be sure.
> 
> 
>>And from an interface perspective, a caller of a function like lookup_one_len
>>should never have to worry about the implementation of the underlying
>>filesystem, or even have to know or care what the filesystem is.
>>
>>>I thought that this was quite sensible and a fairly simple resolution?
>>
>>But it defeats the whole purpose of having an automounter.
> 
> 
> Yep. Sigh.
> 
> But there's more. The Linux autofs implementation lacks some crucial 
> features which I really want to add.
> 
> I posted an RFC to LKML and had no interest but, since it relates to this 
> issue, I'd really appreciate it if you could give it a quick read and 
> perhaps help out a bit with your thoughts.
> 
> http://themaw.net/direct.txt
> 
> The issue that comes up with this is that, for this to work the fs would 
> have to do
> 
> if (follow_link inode method is defined)
> 	call follow_link
> 
> where as 
> 
> if (S_ISLNK(...))
> 	call follow_link
> 
> won't work.
> 
> Of course the other way to do it would be to add another method but that 
> would complicate an already busy link_path_walk. I don't think people 
> would agree with that.
> 
> The side affects of setting the link bit would have undesirable 
> consequences.
> 
> Ideas?
> 
I have looked at your proposal quickly and it seems reasonable on its 
surface.  I have implemented a stackable filesystem, you are wise to 
want to avoid doing so.  I would need to give it some more thought.  But 
  since I will be on holiday until Monday, it is not going to happen soon.

Regards,
Will