linux-kernel - Re: bio pages with zero page reference count

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Date:	Mon, 18 Dec 2006 22:53:43 +0000
From:	Christoph Hellwig <hch@...radead.org>
To:	support@...aid.com
Cc:	linux-kernel@...r.kernel.org, Greg KH <greg@...ah.com>,
	boddingt@...usnet.com.au, Andrew Morton <akpm@...l.org>,
	bugme-daemon@...zilla.kernel.org,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: bio pages with zero page reference count

On Mon, Dec 18, 2006 at 05:21:09PM -0500, Ed L. Cashin wrote:
> (This email is a followup to "Re: [PATCH 2.6.19.1] fix aoe without
> scatter-gather [Bug 7662]".)
> 
> On Mon, Dec 18, 2006 at 12:53:00PM -0500, Ed L. Cashin wrote:
> ...
> > This patch eliminates the offset data on cards that don't support
> > scatter-gather or have had scatter-gather turned off.  There remains
> > an unrelated issue that I'll address in a separate email.
> 
> After fixing the problem with the skb headers, we noticed that there
> were still problems when scatter gather wasn't in use.  XFS was giving
> us bios that had pages with a reference count of zero.
> 
> The aoe driver sets up the skb with the frags pointing to the pages,
> and when scatter gather isn't supported and __pskb_pull_tail gets
> involved, put_page is called after the data is copied from the pages.
> That causes problems because of the zero page reference count.
> 
> It seems like it would always be incorrect for one part of the kernel
> to give pages with a zero reference count to another part of the
> kernel, so this seems like a bug in XFS.
> 
> Christoph Hellwig, though, points out,
> 
>   > It's a kmalloced page.  The same can happen with ext3 aswell, but
>   > only when doing log recovery.  The last time this came up (vs
>   > iscsi) the conclusion was that the driver needs to handle this
>   > case.
> 
> In attempting to find the conversation he was referencing, I only
> found this:
> 
>   Subject: tcp_sendpage and page allocation lifetime vs. iscsi
>   Date: 2005-04-25 17:02:59 GMT
>   http://article.gmane.org/gmane.linux.kernel/298377
> 
> If anyone has a better reference, I'd like to see it.

I searched around a little bit and found these:

	http://groups.google.at/group/open-iscsi/browse_frm/thread/17fbe253cf1f69dd/f26cf19b0fee9147?tvc=1&q=kmalloc+iscsi+%22christoph+hellwig%22&hl=de#f26cf19b0fee9147
	http://www.ussg.iu.edu/hypermail/linux/kernel/0408.3/0061.html

But that's not the conclusion I was looking for.  
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