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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 09/01/2011 12:42 AM, 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.

That's a good point.  (Maybe Xen should have generated pagefaults when
hypercall arg pointers are bad...)

> 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.

There's already a wrapper: xen_alloc_vm_area(), which is just a
#define.  But we could easily add a sync_all to it (and use it in
netback, like we do in grant-table and xenbus).

    J

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