Re: (reiserfs) Re: NFSv4 ACLs (was: ...ACL's and reiser...)

[Hans Reiser]

There is a completely different way of implementing full inheritance that is
efficient, and defaults to the same old behavior where it is unused.

To minimize unnecessary flamage, I'll describe it only after
I can assign a programmer to work on it, though it is not a very deep
or profound solution.

Hans

[e-mail] wrote:
> 
>    Date: Sun, 06 Aug 2000 15:52:19 +0200
>    From: Xuan Baldauf <[e-mail]>
> 
>    Tytso does not like the idea. And after some thinking about the issue, I can
>    understand him. It is because
> 
>    (1) resolving from the bottom to the top might be more costly than
>    static ACLs if no path component checking is needed anymore (path
>    component checking is UNIX's Achilles Heel compared to NT) and (2)
>    there is no definite parent for files with n_link>1
> 
> The difference here is that NT always does path resolution from the
> root, whereas in Unix, we do path resolution of relative pathnames
> (i.e., those whose pathname don't start with a '/') relative to the
> current working directory.
> 
> So doing full inheritable ACL's will force the Unix permissions checking
> model to be as SLOW as NT.  (Consider how many relative pathnames are
> used when doing a kernel compile, for example.)  And oh by the way, it's
> not just me who doesn't like this; most of the folks I chatted to at the
> OLS thought it was also an insane idea, including sct and alan, and a
> number of others.  I very much doubt something like this would ever go
> into the VFS core.  (If a filesystem wants to do this in their
> filesystem dependent code, and trash their Andrew benchmark scores, they
> can feel free to do so.  :-)
> 
>    (1) can be circumvented by having a (persistent) ACL change cache in
>    reiserfs. When accessing a file, the cache (usually empty) is checked
>    against wether the entry in the cache applies to a particular
>    file. If the cache entries do not apply to the file, the files ACL is
>    checked. This way you can always have inheriting ACLs, make ACL
>    changes of millions of files atomic. (Changes place an entry in the
>    cache, and a background thread changes the resolved ACL entries in
>    every file.)
> 
> What are you storing in the cache?  If you are storing the entry for
> each directory, you still have to do the full pathname resolution to
> find all of the parent directories.  If you are storing cache entries
> index by inode number, then each time there is an ACL change, you have
> to flush the entire ACL cache.  Also, this doesn't help you if you're
> doing something like a kernel compile, since you still have to do the
> full pathname resolution for every new file that you've never seen
> before.  (Which in the case of the kernel compile, happens a lot.)
> 
>                                                 - Ted
> 
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