linux-ext4 - Re: 2.6.25-git2: BUG: unable to handle kernel paging request at ffffffffffffffff

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Date:	Mon, 21 Apr 2008 10:30:19 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org
Subject: Re: 2.6.25-git2: BUG: unable to handle kernel paging request at
 ffffffffffffffff

On Mon, 21 Apr 2008, Paul E. McKenney wrote:
> 
> I did take a quick look for improperly freeing dentries -- unhashed
> dentries are freed directly, so if there is a code path that somehow
> unhashes dentries and then d_free()s them without a grace period, we
> have a problem.

No, not even then.

We *always* unhash the dentries before freeing them, but we very 
consciously use "hlist_del_rcu()" on them, not "hlist_del_init()".

That, in turn, will mean that the "pprev" pointer will still be set, so 
the "hlist_unhashed()" thing will *not* trigger.

IOW, when we do that direct-free with:

	if (hlist_unhashed(&dentry->d_hash))
		__d_free(dentry);

the "hlist_unhashed()" will literally guarantee that i has *never* been on 
a hash-list at all!

(If you want to test whether it is currently unhashed or not, you actually 
have to use "d_unhashed()" on the dentry under the dentry lock, which 
tests the DCACHE_UNHASHED bit).

Of course, there could be some bug in there, but the thing is, none of 
this has even changed in a long time, certainly not since 2.6.25. Which is 
why I think the dcache code is all fine, and the bug comes from somewhere 
else corrupting the data structures.

		Linus
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