Re: [regression] Bug 221745 - PV DomU ballooning driver init issue

Hello

I will try this tomorrow !

Regards

Le 27 juillet 2026 18:51:15 GMT+02:00, "Roger Pau Monné" <[email protected]> a écrit :
>On Mon, Jul 27, 2026 at 10:42:25AM +0200, Thorsten Leemhuis wrote:
>> Hi Roger! FYI, it seems your change 0949c646d64697 ("Partial revert
>> "x86/xen: fix balloon target initialization for PVH dom0"") [v7.0-rc1]
>> caued a regression. For details see
>> https://bugzilla.kernel.org/show_bug.cgi?id=221745
>> 
>> To quote the initial comment:
>> 
>> > Since commit db8c4b1 (Partial revert "x86/xen: fix balloon target initialization for PVH dom0"), xen ballooning fails to init correctly on domU (on amd64 arch) with the following configuration:
>> > 
>> > -- xl.cfg
>> > ...
>> > memory      = '2048'
>> > maxmem      = '4096'
>> > ...
>> > --
>> > 
>> > -- domU balloon dmesg (init fails)
>> > [    0.908337] xen:balloon: Initialising balloon driver
>> > [    0.940721] Extra pages underflow current target
>
>Yeah, I'm afraid the original fix commit didn't fully fix the issues,
>so there's the following change (chunk below) that's needed
>additionally to cope with memory != maxmem on PV guests.
>
>I've done some basic testing and it seems to solve the issue, could
>you give it a spin also?  (just to make sure I'm not missing anything
>else).
>
>Thanks, Roger.
>---
>diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>index e7f1d4ca6d75..c20a1ff8292d 100644
>--- a/drivers/xen/balloon.c
>+++ b/drivers/xen/balloon.c
>@@ -703,19 +703,26 @@ static int __init balloon_add_regions(void)
> 			balloon_append(pfn_to_page(pfn));
> 
> 		/*
>-		 * Extra regions are accounted for in the physmap, but need
>-		 * decreasing from current_pages and target_pages to balloon
>-		 * down the initial allocation, because they are already
>-		 * accounted for in total_pages.
>+		 * For HVM domains: extra regions are accounted for in the
>+		 * physmap, but need decreasing from current_pages and
>+		 * target_pages to balloon down the initial allocation, because
>+		 * they are already accounted for in total_pages.
>+		 *
>+		 * For PV domains: extra regions are not accounted for in the
>+		 * initial memory target, and hence need adding to the stats as
>+		 * additional unpopulated regions.
> 		 */
> 		pages = extra_pfn_end - start_pfn;
>-		if (pages >= balloon_stats.current_pages ||
>-		    pages >= balloon_stats.target_pages) {
>+		if (xen_pv_domain()) {
>+			balloon_stats.total_pages += pages;
>+		} else if (pages >= balloon_stats.current_pages ||
>+		           pages >= balloon_stats.target_pages) {
> 			WARN(1, "Extra pages underflow current target");
> 			return -ERANGE;
>+		} else {
>+			balloon_stats.current_pages -= pages;
>+			balloon_stats.target_pages -= pages;
> 		}
>-		balloon_stats.current_pages -= pages;
>-		balloon_stats.target_pages -= pages;
> 	}
> 
> 	return 0;