Re: [autofs] [RFC] Towards a Modern Autofs

[Mike Waychison <Michael.Waychison@Sun.COM>]

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H. Peter Anvin wrote:
> Ian Kent wrote:
> 
>>
>> If wildcard map entries are not in autofs v3 then Jeremy implemented this
>> in v4.
>>
> 
> v3 has had wildcard map entries and substitutions for a very, very, very 
> long time... it was a v2 feature, in fact.
> 
>> And yes the host map is basically a program map and that's all. Worse, as
>> pointed out in the paper it mounts everything under it. This is a source
>> of stress for mount and umount. I have put in a fair bit of time on ugly
>> hacks to work around this. This same problem is also evident in startup
>> and shutdown for master maps with a good number of entries (~50 or more).
>> A consequence of the current multiple daemon approach.
> 
> 
> This is why one wants to implement a mount tree with "direct mount 
> pads"; which also means keeping some state in the daemon.
> 
> For example, let's say one has a mount tree like:
> 
> /foo        server1:/export/foo \
> /foo/bar    server1:/export/bar \
> /bar        server2:/export/bar
> 
> ... then you actually have four diffenent filesystems involved: first, 
> some kind of "scaffolding" (this can be part of the autofs filesystem 
> itself or a ramfs) that hold the "foo" and "bar" directories, and then 
> foo, foo/bar, and bar.
> 
> Consider the following implementation: when one encounters the above, 
> the daemon stashes this away as an already-encountered map entry (in 
> case the map entries change, we don't want to be inconsistent), creates 
> a ramfs for the scaffolding, creates the "foo" and "bar" subdirectories 
> and mount-traps "foo" and "bar".  Then it releases userspace.  When it 
> encounters an access on "foo", it gets invoked again, looks it up in its 
> "partial mounts" state, then mounts "foo" and mount-traps "foo/bar", 
> then releases userspace.
> 
> In many ways this returns to the simplicity of the autofs v3 design 
> where the atomicity constraints where guaranteed by the VFS itself, *as 
> long as* mount traps can be atomically destroyed with umounting the 
> underlying filesystem.
> 

Great!

This is exactly what I found when looking into the situation.  However, 
namespaces still break automounting unless you can rid yourself of the 
daemon.  Move events into call_usermodehelper calls in current's 
namespace and maintain what little state you need as a set of tokens.


-- 
Mike Waychison
Sun Microsystems, Inc.
1 (650) 352-5299 voice
1 (416) 202-8336 voice
mailto: Michael.Waychison@Sun.COM
http://www.sun.com

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