Re: [Xen-devel] [PATCH v5 12/22] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/02/2015 10:52 AM, Julien Grall wrote:
> On 02/10/15 15:31, Julien Grall wrote:
> > Hi David,
> >
> > On 02/10/15 15:09, David Vrabel wrote:
> > > On 30/09/15 11:45, Julien Grall wrote:
> > > > For ARM64 guests, Linux is able to support either 64K or 4K page
> > > > granularity. Although, the hypercall interface is always based on 4K
> > > > page granularity.
> > > >
> > > > With 64K page granularity, a single page will be spread over multiple
> > > > Xen frame.
> > > >
> > > > To avoid splitting the page into 4K frame, take advantage of the
> > > > extent_order field to directly allocate/free chunk of the Linux page
> > > > size.
> > > >
> > > > Note that PVMMU is only used for PV guest (which is x86) and the page
> > > > granularity is always 4KB. Some BUILD_BUG_ON has been added to ensure
> > > > that because the code has not been modified.
> > > This causes a BUG() in x86 PV guests when decreasing the reservation.
> > >
> > > Xen says:
> > >
> > > (XEN) d0v2 Error pfn 0: rd=0 od=32753 caf=8000000000000001
> > > taf=7400000000000001
> > > (XEN) memory.c:250:d0v2 Bad page free for domain 0
> > >
> > > And Linux BUGs with:
> > >
> > > [   82.032654] kernel BUG at
> > > /anfs/drall/scratch/davidvr/x86/linux/drivers/xen/balloon.c:540!
> > >
> > > Which is a non-zero return value from the decrease_reservation hypercall.
> > >
> > > The frame_list[] has been incorrectly populated.  The below patch fixes
> > > it for me.  Please test as well.
> FIY, I've just tested with the patch on ARM64 and I haven't see any issue.

I had a quick one-off test and this fixes it on x86. I'll schedule it 
for the overnight run too.
-boris


> > Sorry for the breakage, I think I haven't spot the bug on my board
> > because most the PV drivers are allocating one balloon page at the time
> > by default.
> >
> > This patch looks valid to me. i was resetting and incremented for each
> > loop on an early version. Although I dropped it by mistake when I use a
> > different way to decrease the reservation.

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