From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <Ian.Campbell@citrix.com>
Subject: Re: [PATCH 2/2] xen/privcmd: add PRIVCMD_MMAPBATCH_V2
 ioctl
Date: Fri, 31 Aug 2012 08:02:12 +0100
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To: Andres Lagar-Cavilla <andreslc@gridcentric.ca>
Cc: "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, David Vrabel <david.vrabel@citrix.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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On Thu, 2012-08-30 at 19:29 +0100, Andres Lagar-Cavilla wrote:
> On Aug 30, 2012, at 1:04 PM, David Vrabel wrote:
> > You can't access user space pages here while holding
> > current->mm->mmap_sem.  I tried this and it would sometimes deadlock in
> > the page fault handler.
> > 
> > access_ok() only checks if the pointer is in the user space virtual
> > address space - not that a valid mapping exists and is writable.  So
> > BUG_ON(__put_user()) should not be done.
> 
> Very true. Thanks for the pointer. Clearly the reason for the gather_array/traverse_pages structure.

/me has flashbacks to the mammoth debugging session which led to
http://xenbits.xen.org/hg/linux-2.6.18-xen.hg/rev/043dc7488c11.

Ian.