linux-kernel - Re: [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 11:18:58 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.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,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	stefano.stabellini@...citrix.com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 12/22] xen/balloon: Don't rely on the page granularity
 is the same for Xen and Linux



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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