Re: [Xen-devel] [PATCH] direct_remap_pfn_range vm_flags fix

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 8 Feb 2006, at 08:28, Chris Wright wrote:

> direct_remap_pfn_range() does not properly mark vma with VM_PFNMAP.
> This triggers improper reference counting on what rmap thought was
> a normal page, and a subsequent BUG() such as:
It isn't really proper for direct_remap_pfn_range() to set VM_PFNMAP. 
Properly that function should actually be called remap_mfn_range(). 
vm_pgoff is an MFN, and what is mapped is a contiguous sequence of 
MFNs.
So the PFNMAP checks in vm_normal_page() do not work, since pte_pfn() 
will not return a contiguous sequence of PFNs starting from vm_pgoff -- 
the MFNs are contiguous, not the PFNs. What saves you currently is the 
cow_mapping() check in vm_normal_page(). If we created private mappings 
of any guest-local memory via direct_remap_pfn_range(), we would still 
crash.
Possible proper fixes:
 1. Don't map local memory via that interface -- only xenstored does 
this, when mapping dom0's xenbus page. This could be changed.
 2. Add a _PAGE_DIRECTMAP flag, just for Xen, that indicates 
unrefcounted PTEs.
 -- Keir


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