Re: [PATCH v5 12/22] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux
From: David Vrabel
Date: Fri Oct 02 2015 - 10:09:26 EST
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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