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 15:09:08 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Julien Grall <julien.grall@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<linux-arm-kernel@...ts.infradead.org>, <ian.campbell@...rix.com>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Wei Liu <wei.liu2@...rix.com>
Subject: Re: [PATCH v5 12/22] xen/balloon: Don't rely on the page granularity
 is the same for Xen and Linux

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.

--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -504,9 +504,10 @@ static enum bp_state decrease_reservation(unsigned
long nr_pages, gfp_t gfp)
 	 * Setup the frame, update direct mapping, invalidate P2M,
 	 * and add to balloon.
 	 */
+	i = 0;
 	list_for_each_entry_safe(page, tmp, &pages, lru) {
 		/* XENMEM_decrease_reservation requires a GFN */
-		frame_list[i] = xen_page_to_gfn(page);
+		frame_list[i++] = xen_page_to_gfn(page);

 #ifdef CONFIG_XEN_HAVE_PVMMU
 		/*

David
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