linux-kernel - Re: [PATCH 2/2] xen: before ballooning hotplugged memory, set frames to invalid

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Date:	Thu, 19 Mar 2015 19:40:57 +0100
From:	Daniel Kiper <daniel.kiper@...cle.com>
To:	Juergen Gross <jgross@...e.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, david.vrabel@...rix.com,
	boris.ostrovsky@...cle.com
Subject: Re: [PATCH 2/2] xen: before ballooning hotplugged memory, set frames
 to invalid

On Thu, Mar 19, 2015 at 06:22:06PM +0100, Juergen Gross wrote:
> On 03/19/2015 05:21 PM, Daniel Kiper wrote:
> >On Thu, Mar 19, 2015 at 03:31:02PM +0100, Juergen Gross wrote:
> >>Commit 25b884a83d487fd62c3de7ac1ab5549979188482 ("x86/xen: set
> >>regions above the end of RAM as 1:1") introduced a regression.
> >>
> >>To be able to add memory pages which were added via memory hotplug to
> >>a pv domain, the pages must be "invalid" instead of "identity" in the
> >>p2m list before they can be added.
> >>
> >>Suggested-by: David Vrabel <david.vrabel@...rix.com>
> >>Signed-off-by: Juergen Gross <jgross@...e.com>
> >>---
> >>  drivers/xen/balloon.c | 13 +++++++++++--
> >>  1 file changed, 11 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> >>index 0b52d92..52e331f 100644
> >>--- a/drivers/xen/balloon.c
> >>+++ b/drivers/xen/balloon.c
> >>@@ -221,15 +221,24 @@ static bool balloon_is_inflated(void)
> >>
> >>  static enum bp_state reserve_additional_memory(long credit)
> >>  {
> >>-	int nid, rc;
> >>+	int nid, rc = 0;
> >>  	u64 hotplug_start_paddr;
> >>  	unsigned long balloon_hotplug = credit;
> >>+	unsigned long pfn;
> >>
> >>  	hotplug_start_paddr = PFN_PHYS(SECTION_ALIGN_UP(max_pfn));
> >>  	balloon_hotplug = round_up(balloon_hotplug, PAGES_PER_SECTION);
> >>  	nid = memory_add_physaddr_to_nid(hotplug_start_paddr);
> >>
> >>-	rc = add_memory(nid, hotplug_start_paddr, balloon_hotplug << PAGE_SHIFT);
> >>+	for (pfn = PFN_DOWN(hotplug_start_paddr);
> >>+	     !rc && pfn < PFN_DOWN(hotplug_start_paddr) + balloon_hotplug;
> >>+	     pfn++)
> >>+		if (!set_phys_to_machine(pfn, INVALID_P2M_ENTRY))
> >
> >rc = set_phys_to_machine(pfn, INVALID_P2M_ENTRY)?
>
> Not really. set_phys_to_machine returns false on failure...
>
> >
> >>+			rc = 1;
> >
> >I do not think that this stuff is needed for HVM or PVH guests.
>
> True.
>
> >
> >>+	if (!rc)
> >>+		rc = add_memory(nid, hotplug_start_paddr,
> >>+				balloon_hotplug << PAGE_SHIFT);
> >>
> >>  	if (rc) {
> >>  		pr_warn("Cannot add additional memory (%i)\n", rc);
> >
> >It will be nice to know what part of infrastructure failed.
> >Could you create separate pr_warn() message for set_phys_to_machine()?
>
> Value 1 for rc is the indicator for that case.

Well... Personally I prefer explicit messages telling what happened than
something which requires digging in a code to understand a problem.
Additionally, I think that we should use negative numbers (as David
suggested) to signal an error. Most of kernel stuff work in that way.

Daniel
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