Re: still nfs problems [Was: Linux 2.6.37-rc8]

From: George Spelvin
Date: Thu Dec 30 2010 - 22:18:03 EST


> Uncached_readdir is not really a problem. The real problem is
> filesystems that generate "infinite directories" by producing looping
> combinations of cookies.
>
> IOW: I've seen servers that generate cookies in a sequence of a form
> vaguely resembling
>
> 1 2 3 4 5 6 7 8 9 10 11 12 3...
>
> (with possibly a thousand or so entries between the first and second
> copy of '3')
>
> The kernel won't loop forever with something like that (because
> eventually filldir() will declare it is out of buffer space), but
> userland has a halting problem: it needs to detect that every
> sys_getdents() call it is making is generating another copy of the
> sequence associated with '4 5 6 7 8 9 10 11 12 3'...

Huh? This is not only an easy problem, it's a well-known problem.
http://en.wikipedia.org/wiki/Cycle_detection

Here's Brent's algorithm:

n = 0;
saved_cookie = <invalid>
For each cookie {
if (n && cookie == saved_cookie)
die("Loop detected!");
if (++n is a power of 2)
saved_cookie = cookie;
}

You can tweak the performance with other exponentially-growing
functions, saving k > 1 old cookies for comparison, etc., but the
above will work very well.
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