From: Frank Filz <ffilzlnx@us.ibm.com>
Subject: Re: Crash in dec_zone_page_state when nfs_page req is freed
Date: Mon, 09 Oct 2006 11:30:41 -0700
Message-ID: <1160418641.3376.103.camel@dyn9047022153>
References: <1158623736.3376.8.camel@dyn9047022153>
	<1158636275.5896.14.camel@lade.trondhjem.org>
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Cc: NFS List <nfs@lists.sourceforge.net>
To: Trond Myklebust <trond.myklebust@fys.uio.no>
In-Reply-To: <1158636275.5896.14.camel@lade.trondhjem.org>
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On Mon, 2006-09-18 at 23:24 -0400, Trond Myklebust wrote:
> On Mon, 2006-09-18 at 16:55 -0700, Frank Filz wrote:
> > I am seeing a crash in dec_zone_page_state when called from 
> > 
> > static void nfs_cancel_commit_list(struct list_head *head)
> > {
> > 	struct nfs_page *req;
> > 
> > 	while(!list_empty(head)) {
> > 		req = nfs_list_entry(head->next);
> > 		nfs_list_remove_request(req);
> > 		nfs_inode_remove_request(req);
> > 		nfs_clear_page_writeback(req);
> > 		dec_zone_page_state(req->wb_page, NR_UNSTABLE_NFS);
> > 	}
> > }
> > 
> > I see this was somewhat recently added. It appears that
> > nfs_clear_page_writeback has resulted in the req being freed. We are
> > running with CONFIG_SLAB_DEBUG on which poisons memory with repeated
> > 0x6b bytes when freed so the subsequent reference to req results in a
> > bad wb_page pointer.
> 
> Does the attached patch fix it for you?

I've had a chance to do some testing of this. It turns out that
nfs_inode_remove_request() will set wb_page to NULL, so it looks like
the call to dec_zone_page_state needs to be moved up before
nfs_inode_remove_request(). I'm testing that right now, but I start to
wonder exactly what the dec_zone_page_state is doing in this case, I
know that was a recent addition.

Frank Filz


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