Re: [Xen-devel] Kernel bug from 3.0 (was phy disks and vifs timing out in DomU)

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 01/09/11 15:23, Konrad Rzeszutek Wilk wrote:
> On Thu, Sep 01, 2011 at 08:42:52AM +0100, Ian Campbell wrote:
>> On Wed, 2011-08-31 at 18:07 +0100, Konrad Rzeszutek Wilk wrote:
>>> On Wed, Aug 31, 2011 at 05:58:43PM +0100, David Vrabel wrote:
>>>> On 26/08/11 15:44, Konrad Rzeszutek Wilk wrote:
>>>>>
>>>>> So while I am still looking at the hypervisor code to figure out why
>>>>> it would give me [when trying to map a grant page]:
>>>>>
>>>>> (XEN) mm.c:3846:d0 Could not find L1 PTE for address fbb42000
>>>>
>>>> It is failing in guest_map_l1e() because the page for the vmalloc'd
>>>> virtual address PTEs is not present.
>>>>
>>>> The test that fails is:
>>>>
>>>> (l2e_get_flags(l2e) & (_PAGE_PRESENT | _PAGE_PSE)) != _PAGE_PRESENT
>>>>
>>>> I think this is because the GNTTABOP_map_grant_ref hypercall is done
>>>> when task->active_mm != &init_mm and alloc_vm_area() only adds PTEs into
>>>> init_mm so when Xen looks in the page tables it doesn't find the entries
>>>> because they're not there yet.
>>>>
>>>> Putting a call to vmalloc_sync_all() after create_vm_area() and before
>>>> the hypercall makes it work for me.  Classic Xen kernels used to have
>>>> such a call.
>>>
>>> That sounds quite reasonable.
>>
>> I was wondering why upstream was missing the vmalloc_sync_all() in
>> alloc_vm_area() since the out-of-tree kernels did have it and the
>> function was added by us. I found this:
>> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=ef691947d8a3d479e67652312783aedcf629320a
>>
>> commit ef691947d8a3d479e67652312783aedcf629320a
>> Author: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
>> Date:   Wed Dec 1 15:45:48 2010 -0800
>>
>>     vmalloc: remove vmalloc_sync_all() from alloc_vm_area()
>>     
>>     There's no need for it: it will get faulted into the current pagetable
>>     as needed.
>>     
>>     Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
>>
>> The flaw in the reasoning here is that you cannot take a kernel fault
>> while processing a hypercall, so hypercall arguments must have been
>> faulted in beforehand and that is what the sync_all was for.
>>
>> It's probably fair to say that the Xen specific caller should take care
>> of that Xen-specific requirement rather than pushing it into common
>> code. On the other hand Xen is the only user and creating a Xen specific
>> helper/wrapper seems a bit pointless.
> 
> Perhaps then doing the vmalloc_sync_all() (or are more precise one:
> vmalloc_sync_one) should be employed in the netback code then?
> 
> And obviously guarded by the CONFIG_HIGHMEM case?

Perhaps. But I think the correct thing to do initially is revert the
change and then look at possible improvements.  Particularly as the fix
needs to be a backported to stable.

David


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