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 15:31:45 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	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>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<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

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.

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.

Regards,

-- 
Julien Grall
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