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

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Date:	Fri, 2 Oct 2015 16:19:31 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Julien Grall <julien.grall@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	Wei Liu <wei.liu2@...rix.com>, <ian.campbell@...rix.com>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH v5 12/22] xen/balloon: Don't rely on the page
 granularity is the same for Xen and Linux

On 02/10/15 15:52, 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.

Thanks, I've folded it in.

Boris, can we get another test run on the for-linus-4.4 branch, please?

David
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